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"I think an almost unbelievable amount of false philosophy has arisen through not realizing what 'existence' means." (p. 234)
From: Bertrand Russell, Logic and Knowledge. Essays 1901-1950, New York: Macmillan 1956.
"Philosophical discussion of the notion of existence, or being, has centered on two main problems which have not always been very clearly distinguished. First, there is the problem of what we are to say about the existence of fictitious objects, such as centaurs, dragons, and Pegasus; second, there is the problem of what we are t o say about the existence of abstract objects, such as qualities, relations, and numbers. Both problems have tempted philosophers to say that there are inferior sorts of existence a s well as the ordinary straightforward sort, and they therefore often suggest that we use the word "being" to cover both kinds but restrict "existence" to "being" of the common, non-fictitious, non-abstract sort. (Sometimes the term "reality" is proposed for "existence" or for "being.") T he problems of fiction and abstraction are different, how ever, for there are both real and fictitious abstractions. For example, the integer between two and four is real, but the integer between two and three is fictitious. On the other hand, there are both concrete and abstract fictions; for example, the winged horse of Bellerophon and the integer between two and three. Accordingly, philosophers have often dealt with the two problems in quite different ways and perhaps ought to do so.
While these are the two main problems, there are others, f or example, that of what we are to say of the being of objects which have not yet begun, or have now ceased, to exist. The history of this subject, moreover, has been tangled with theological issues, to which it will be necessary to refer at certain points." (p. 493)
From: Paul Edwards (ed.), Encyclopedia of Philosophy, New York: Macmillan 1967, entry Existence, by Arthur Norman Prior; Second edition: Donald M. Borchert (ed.), New York: Thomson Gale 2006, vol. 3, pp. 493-500.
"The Indo-European languages in which all the philosophers from Parmenides to Sartre have written have a verb represented in English by 'be', which some of the time at least does the same work as is done by 'exist'. Everything that can be called philosophy of existence that was written by the Greek philosophers of antiquity was expressed with the help of 'einai', the Greek equivalent of 'be'; and it is impossible to reach any clear understanding of their doctrines without examining how they used this word, and how its synonyms in other languages are used." (Preface, p. VIII-IX) "German and French idioms, which most frequently use 'Es gibt' and 'Il y a' in place of 'There is', seem to show a stronger awareness than English of the difference between existential propositions and propositions ascribing properties to objects. Nevertheless even these languages have forms 'Es ist' and 'Il est', which make use of equivalents of the verb 'be', as English uses 'be' in 'There is' and 'There are', and one would have to look further a field to find languages where there was no possibility of construing an existential judgement as predicating being of an object or objects in the same way as dwelling in Transylvania or coming down the road can be predicated of an object or objects. Latin, as we have seen, has the simple unadorned use of 'est' and 'sunt' as a possible substitution for the verbs 'existit' and 'existunt'. Classical Greek, which lacks any word obviously equivalent to 'exist', is forced to use parts of 'einai', its synonym for 'be', much more widely than the languages we have mentioned, for the expression of existential judgements. Thereby hangs a philosophical story of epic dimensions, a Great Chain of Philosophies of Being." (p. 3)
From: Christopher John Fards Williams, What is Existence?, Oxford: Clarendon Press 1981.
"In the extended discussion of the concept (or concepts) of Being in Greek philosophy from Parmenides to Aristotle, the theme of existence does not figure as a distinct topic for philosophical reflection. My aim here is to defend and illustrate this claim, and at the same time to suggest some of the reasons why it is that the concept of existence does not get singled out as a topic in its own right. Finally, I shall raise in a tentative way the question whether or not the neglect of this topic was necessarily a philosophical disadvantage.
Let me make clear that my thesis is limited to the classical period of Greek philosophy, down to Aristotle. The situation is more complicated in Hellenistic and Neoplatonic thought, (...) I suspect that a careful study of these Greek terms would reveal that even in their usage we find no real equivalent of our concept of existence. In any case, this later terminology (...) plays no part in the formulation of Plato's and Aristotle's ontology, and I shall ignore it here. My general view of the historical development is that existence in the modern sense becomes a central concept in philosophy only in the period when Greek ontology is radically revised in the light of a metaphysics of creation: that is to say, under the influence of Biblical religion. As far as I can see, this development did not take place with Augustine or with the Greek Church Fathers, who remained under the sway of classical ontology. The new metaphysics seems to have taken shape in Islamic philosophy, in the form of a radical distinction between necessary and contingent existence: between the existence of God, on the one hand, and that of the created world, on the other." (p. 7)
To return now to the question with which we began: Why does existence not emerge as a distinct concept in Greek philosophy? In principle the answer is clear. My explanation is that in Greek ontology in its early stages, in Plato and Parmenides, the veridical concept was primary, and the question of Being was the question of "reality" as determined by the concept of truth. Since this conception of reality is articulated in Plato by copula sentences of the form "X is Y," it turns out that even the concept of existence gets expressed in this predicative form: as we have seen, Platonic Greek for "X exists" is "X is something" . In the scheme of categories which Aristotle takes as the starting point for his own investigation of being, this same predicative pattern serves as the primary device for analyzing what there is, and for showing how the various kinds of being are related to one another. So it is naturally the theory of predication, and not the concept of existence, which becomes the central and explicit theme of Aristotle's metaphysics, as it was the implicit theme of Plato's discussion of Being in the Sophist. (p. 15).
From: Charles H. Kahn, Why Existence does not Emerge as a Distinct Concept in Greek Philosophy, in: Parviz Morewedge (ed.), Philosophies of Existence. Ancient and Medieval, New York: Fordham University Press 1982, pp. 7-17. (Reprinted in C. H. Kahn. Essays on Being, New York: Oxford University Press 2009, pp. 62-74.)
"The terminology for existence is much more complicated, and I can only give a rough sketch of the problem. We have first to consider how ὑπάρχειν (hyparkein) comes to be used as a synonym for εἶναι (einai) in its "existential" use, and then to follow the history of existere as the Latin rendering of ὑπάρχειν in this sense. Either topic could supply a separate monograph.
ὑπάρχειν originally means "to make a beginning", "to take the initiative", "to take the first step (in doing so-and-so)", e.g. to begin a guest-friendship (in the earliest occurrence of the verb, Odyssey 24.286) or to initiate hostilities (...)
Thus before ὑπάρχειν becomes specialized as a verb of "existence" we find it used in a predicative construction as an expressive equivalent for εἶναι as copula verb.
It is, however, not this copula use but the more frequent construction with the dative that accounts for the first technical use of the verb in philosophy: the use in which it expresses in logical terms the attributive relation which is normally expressed in grammatical form by the copula. Instead of "A is B" Aristotle prefers to say τὁ β τὸ ἀ ὑπάρχειν B belongs to A" (Prior Analytics 25a,5 and throughout). Hence τα ὑπάρχοντα attributes (of a subject) " e.g. at De Interpretatione 16b 10. (And see Bonitz, Index Aristotelicum 789a29-b2; compare the more non-technical use ibid. a12-28.) Since "what belongs to a thing" includes not only its accidents but also essential or substantial attributes in the first category, ὑπάρχειν is said in as many ways as εἶναι, i.e. in as many ways as there are categories or combinations of categories (Pr. An. 48b2-4, 49a6-9). (...)
Apart from this technical use in logic and grammar, the most common meaning of ὑπάρχειν in later Greek seems to be that which we render as 'to exist' or "'to be real'. (This occasionally leads to rather ludicrous confusion, when a late commentator can no longer distinguish between Aristotle's technical sense and his own ordinary use of ὑπάρχειν. (...).
It should be pointed out that although this use of ὑπάρχειν for real existence (in contrast to a mere word or an imaginary object) seems to be the dominant use in late Greek philosophy, the corresponding verb may still be construed both with paralocative and nominal predicates, as we can see from Sextus' discussion of the existence of the gods, e.g. IX-I43 (...).
And the same predicative construction is normal for the corresponding verb exsistere in classical Latin (as will be seen in a moment). In neither case, then, would our familiar contrast between an existential and a copulative verb naturally arise.(...)
I have neither space nor skill to follow the history of exsistere, exsistentia in Latin. I note, however, that like other derivatives of stare, exsistere serves in Varro, Lucretius, and later authors as a stylistic variant for esse, often with the nuance (suggested by ex-) of 'emerge' come into being' 'be produced'. As in the case of ὑπάρχειν this quasi-existential sense of exsistere is fully compatible with the copulative construction: (pecora) quae post tempus nascuntur, fere vitiosa atque inutilia exsistunt. (Varro Rerum rust. II.1.7)
The noun exsistentia seems not be attested before Marius Victorinus and Candidus in the 4th century A.D. It is a learned invention, designed to render ὑπάρχειν; in metaphysical texts where the latter term is distinguished from οὐσια (substantia) as the more general concept, sheer being without categorial determination, while οὐσια presents some determinate form of being, like 'substance' in the first Aristotelian category: Id est exsistentia vel subsistentia vel, si ... dicas ... vel exsistentialitatem vel substantialitatem vel essentialitatem (Adversus Arium III.7.9, cited in Pierre Hadot, Porphyre et Victorinus (Paris, 1968), II p. 29 text 40).
But this terminology was not taken up by Boethius, who apparently preferred esse to exsistere as a rendering of the technical use of ὑπάρχειν for pure, indeterminate being. (1)
As we have seen, Priscian in the 6th century A.D. renders ὑπάρχειν as substantivum. Thus neglected by Boethius and Priscian, the technical use of exsistentia as contrasted with substantia in late Neo-Platonism had no direct impact on early medieval terminology.
Abelard's usage is mixed. He often employs exsistere, exsistens for 'to exist', 'existing (thing)', but rarely uses the abstract noun and then in a rather surprising way: exsistentiae rerum seems to mean '(actual) states of affairs' in contrast to res, the existing thing, whose existence may be expressed by esse. (Dialectica, 154, 11 and 156, 29. Compare Kneale and Kneale, The Development of Logic, p. 206).
Aquinas' usage is even more Boethian: his normal expression for what we call 'existence' is esse or actus essendi. Only with the esse existentiae of Duns Scotus at the end of the thirteenth century do we find existentia firmly established as a technical term contrasted with essentia. Thus the modern terminology of 'existence' seems to derive from Scotus.
What connections (if any) can be traced between Scotus' use of existentia and the technical terminology of Victorinus nearly a millennium earlier, I do not know. In his translations of Proclus, William of Moerbeke renewed the ancient practice of rendering ὑπάρχειν by existentia, (P. O. Kristeller, Journal of Philosophy, 1962, p. 77) and these translations must have had some influence on the shaping of the medieval terminology. But ὑπάρχειν for Proclus is not quite the same either as exsistentia for Victorinus or existentia for Scotus. What role was played here by the concepts and terminology of Islamic philosophy I can only guess. The history of 'existence' seems to consist largely of still unanswered questions.(2)" (pp. 151-155)
(1) Compare they key passage of Boethius' De Hebdomadibus with the corresponding citation from Damascius in P. Hadot "La distinction de l'être et de l'étant dans le 'De Hebdomadibus' de Boèce", Miscellanea Medievalia, 2, (1963), pp. 147 and 151, n. 25. Boethius shows no trace of the exsistentia-substantia distinction we find in Victorinus and Candidus. It has been pointed out (by Graham, below) that Boethius normally renders the substantival τὸ ὂν by the artificial form ens, but sometimes resorts to the more natural Latin form exsistens for the verbal-adjectival use of the Greek participle as predicate or attribute.
(2) For some remarks on the contrast between Arabic and Greek terminology for "being", see A. C. Graham "'Being' in Linguistics and Philosophy", Foundations of Language, 1 (1965), 223 ff. For doctrinal contacts between Avicenna and Duns Scotus on the question of existence, see E. Gilson, L'être et l'essence, pp. 128-131.
From: Charles H. Kahn, On the Terminology for Copula and Existence, in: S. M. Stern, Albert Hourani and Vivian Brown (eds.), Islamic Philosophy and the Classical Tradition. Essays Presented by His Friends and Pupils to Richard Walzer on His Seventieth Birthday, London: Bruno Cassirer 1972, pp. 141-158. (Reprinted in C. H. Kahn. Essays on Being, New York: Oxford University Press 2009, pp. 41-61.)
"The contemporary attempts to determine in a purely logical way the nature of existence, by constructing a logic of existence(1) or a free logic which makes "no assumptions about the existence of the purported designata of its terms, general or singular,"(2) seems to be still premature as long as the problem of existence in its philosophical implications is not adequately resolved. Historically, there are three main problems in logic involving a conception of existence, which clearly show that the logical problem of existence has a philosophical background influencing the "logic of existence" in its technical sense.
The first problem arose in the period of the algebra of logic, when G. Boole (implicitly) and E. Schroeder (explicitly) introduced the concept of the empty class. This innovation resulted in a criticism of the traditional square of opposites and Aristotelian syllogistics, and the conception of the existential import of categorical statements. A further consequence, already recognized in the period of G. Frege and B. Russell, was the interpretation of particular statements as existential statements and of universal statements as hypothetical ones. It was maintained that "all general propositions deny the existence of something or other,"(3) and for these reasons no valid inference of a statement with existential import from a statement without existential import was admitted.
The second problem comprises the "fanciful Russellian analyses of proper names in existential contexts,"(4) with its special theory of descriptions, directed against Meinong's conception that a grammatically correct denoting phrase stands for an object, even if it does not subsist, and a similar view of H. MacColl who assumed two sorts of individuals -- real and unreal.(5) Russell's theory of descriptions is evidently a philosophical reflection of his views concerning the nature of existence as the fundamental problem cf ontology, a technical term introduced for the first time by Chr. Wolff in his Philosophia prima sive Ontologia . . . (1730). For philosophical reasons, Russell attempts to refute the ontological argument of Anselm of Canterbury. In the course of his argument. he adopts both conclusions of Kant's refutation in his Critique of Pure Reason (B 620ff), namely that (1) "all existential propositions are synthetic" and that (2) "Being is obviously not a real predicate; that is, it is not a concept of something which could be added to the concept of a thing." His conviction that "existence is quite definitely not a predicate" is another reason for his introduction of the concept of the existential qualifier.
The third problem, introduced recently by W. V. Quine in his famous dictum, "To be is, purely and simply, to be the value of a variable,"(6) is connected with the contemporary discussion on universals and the attempts to reformulate the language of logic and mathematics in a nominalistic sense.(7)" (pp. 157-158)
(1) See esp. H. S. Leonard, "The Logic of Existence," Philosophical Studies, VII, 49-64 .
(2) K. Lambert, "Free Logic and the Concept of Existence," Notre Dame Journal of Formal Logic, VIII, 138 . Comp. J. Hintikka, "Existential Presuppositions and Existential Commitments," Journal of Philosophy, LVI, 135 .
(3) B. Russell, "The Philosophy of Logical Atomism," Monist, XXIX, 191 .
(4) S. Candlish, "Existence and the Use of Proper Names," Analysis, XXVIII, 157 .
(5) Comp. B. Russell, "On Denoting," Mind, N. S., XIV, 479-493 
(6) W. V. Quine, "On What There Is," Review of Metaphysics, II, 32 .
(7) See, e.g., N. Goodman and W. V. Quine, "Steps Toward a Constructive Nominalism," Journal of Symbolic Logic, XII, 105-22 ; I. M. Bochenski, A. Church, and N. Goodman, The Problem of Universals, (Notre Dame, 1956).
From: Karel Berka, "Existence in Modern Logic", in Cael G. Vaught (ed.), Essays in Metaphysics, University Park: Pennsylvania University Press 1970, pp. 157-174.
[First distinction between essence and existence]
"The primary analysis of the nature of being, its application to numerous things, and an introduction to the exposition of substance.
Being is recognized by reason itself without the aid of definition or description. Since it has no definition it has neither genus nor differenzia because nothing is more general than it.(1) Being does not have a description since nothing is better known than it. It is possible that one can recognize its name in one language by means of another language. Thus, by some means, it is possible to acquaint someone with what is meant by a term like being. For example, if being appears in Arabic, it can be explained in Persian, and one can indicate that it is that from which other things are derived.
In its first division, being is prima facie of two kinds: the one is called substance and the other accident. Accident is that whose being subsists in something else, so hat that being which is complete without it is either active by itself or due to something else. An example of this condition is the whiteness of a cloth. We note that the cloth exists either by itself due to itself, or by means of those things which bring about its being. Whiteness subsists in a dependent manner in it. Whiteness and whatever is analogous to it are called accidents. In this context, the receptable of whiteness is called a subject, although in another context something else is meant by 'subject'. Thus, a substance is that which is not an accident, whose being, moreover, is not in a subject, but is a reality such that the being of that reality that essence are not receptive to another thing having the aforesaid characteristics. One may regard the substance as a receptable which lacks this character. But to be active, this substance needs to be accepted by this receptacle whose reality we t establish later when we clarify its nature. One may regard the substance neither as a receptable nor as being in a receptacle, as we shall also establish subsequently when we explain its being. This, then, is called a 'substance'." (pp. 15-16).
(1) "The term used by ibn Sina to designate 'being' is hasti, a Persian term for which no equivalent appears in his Arabic texts. To be sure, in Shifa' he points out the term wujud (existence) has several meanings: (1) haqiqa (the essence, reality of something, the fact that it exists), and (2) the particular existence of something, and by making these distinctions, he confirms his awareness of the various senses of 'existence', but even in view of these different senses of 'existence', there is still no term in his Arabic works which could render hasti adequately. Although one could attempt to find a Greek equivalent for this term translating it perhaps as tò òn hê òn by which 'being-qua-being' is commonly rendered, one should nevertheless be cautious not to equate this term with ousía for the reason that ousía is sometimes defined as 'substance'. Ibn Sina, however, holds 'being' in the sense of basti to be the most determinable concept. Hence, if we choose to accept the translation of ousía for being, then ibn Sina's views will definitely be at odds with those of many Neo-Platonists, such as Proclus, who states that the One and the Gods are to be regarded as 'supra being' (hyperoûsios) (Elements, p. 105), or Plotinus, who proclaims that the One generates gennetés (being) (Enneads V ). In view of the preceding discussion in this chapter, it is evident that ibn Sina's doctrine disagrees with that of these Greek philosophers, for he asserts that nothing is above being, and whatever exists is a determination of being. However, Aristotle's notion of being-qua-being as tò òn hé òn (Metaphysica, book IV, ch. 1) corresponds to ibn Sina's notion of hasti. Aristotle's position becomes clear in subsequent sections of the Metaphysica when he indicates that mathematics 'cuts off ' a part of being, whereas metaphysics investigates being as being, ignoring those elements of being which are related to it in an accidental manner (i.e. are a determination of being). For a detailed account of this topic, see J. Owens, The Doctrine of Being in the Aristotelian Metaphysics, Toronto, 1953, esp. ch. 7 and 8. In order to achieve greater clarity on this issue and circumvent various ambiguous usages of the term ousía, Owens suggests the use of the term ' entity' as a better translation of it." (pp. 112-113)
From: Parviz Morewdge, The Metaphysica of Avicenna (ibn Sina). A critical translation-commentary and analysis of the fundamental arguments in Avicenna's Metaphysica in the Danish Nama-i-ala i (The Book of Scientific Knowledge), London: Routledge & Kegan Paul 1973.
[Essence and existence according to Suárez]
"To avoid an equivocation in terms and to make it unnecessary later to make distinctions about an essential being, an existential being or a subsistential being or a being of truth in a proposition, I suppose by being we understand the actual existence of things. For essential being, if it is truly distinguished from existence, adds nothing real to the essence itself, but only differs from it in the way it is conceived or signified. Hence, just as the essence of a creature as such, in virtue of its concept, does not say that it would be something actually real with being outside its causes, so the essential being as by standing precisely in this, does not express an actual being by which an essence outside it causes would be constituted in act. For if to be actual in this latter way is not of the essence of the creature, neither will it be able to pertain to its essential being. Hence, being of a creature as such will prescind of itself from actual being outside its causes by which a created thing comes to be beyond nothing, by which name we designate actual existential being. But subsistential being is also more contracted than existential being, for the latter is common to substance and accidents. The former is proper to substance. Besides, subsistential being (as I suppose from what is to be proved below) is something distinct from the existential being of a substantial created nature and separable from it, because it does not constitute a nature in the order of actual entity, which pertains to existence. Now the being of truth in a pro position of itself is not a real and intrinsic being, but it is an objective being in the intellect as it is composing; hence it belongs also to privations. For we say, accordingly: Blindness is or A man is blind, as Aristotle discusses at greater length in book 5 of Metaphysics, chapter seven. Hence, the discussion is about created existence concerning which, furthermore, we suppose that it is something real and intrinsic to an existing thing; this seems self-evident. For through existence a thing is understood to be something in the nature of things. Therefore, it is necessary that existence be both something real and intrinsic, that is, within the existing thing itself. For a thing cannot be existing by some extrinsic denomination or some being (ens) of reason. Other wise, how would existence constitute a real being (ens) in act and beyond nothing?" (p. 45).
"I say thirdly: that being by which the essence of a creature is formally constituted in essential actuality is the true existential being. (...) Now this statement is proved in a variety of ways. First, because this being, understood precisely, is sufficient for the truth of this statement with a second adjacent: essence is. Hence, that being is true existence. The consequence is clear, for according to the common meaning and human conception, the is of a second adjacent, is not divorced from time. But it signifies being in act in the realm of things, which all of us understand by the name existence or by existential being. You will say that the "is" is always said truly of an actual essence, yet not formally because of the actuality of an essence, nor on account of that being by which it is formally constituted in such actuality, but because it never has this being without existence, although distinct from such an essential being or actuality. But against this retort the antecedent of the argument given is proved. For, by this actual essential being, taken formally and precisely, such an essence is a being (ens) in act and distinguished from a being (ens) in potency. Hence, by virtue of that being, such an essence is, for the inference is correct: it is a being (ens) in act; therefore it is. For to be a being (ens) in act does not reduce the character of being (ens) which includes the verb "is". So, even if we grant that this actual essential being depends on a further limit or act, as on a necessary condition or something of this sort, still that very being will formally constitute a being (ens) in act and will distinguish the latter from a being (ens) in potency. Thus, by virtue of that being a thing is truly and absolutely said to be, just as an accident by virtue of its being is said to be a being (ens) in act and to be absolutely, even though that being requires an inherence in a subject so that, without it, it could not exist naturally." (p. 74).
From: Francisco Suárez, De essentia entis finiti ut tale est et de illius esse eorumque distinctione. (Disputatio XXXI) - (Translated as: On the Essence of Finite Being as Such, on the Existence of that Essence and Their Distinction, by Norman J. Wells, Milwaukee: Marquette University Press 1983).
"To know what existence is, all we have to do is to understand the meaning of the word, for it tells us at once what the thing is which the word stands, for, in so far as we can know it. There is no need here for a definition, which would confuse rather than clarify the issue" (AT X, 525; CSM II, 418).
From: René Descartes, Recherche de la Verité, in: Charles Adam, Paul Tannery (eds.), Oeuvres de Descartes, Paris: Vrin, 1964-1976 (12 volumes; referred as 'AT'); translated as: The Search of Truth, in: J. Cottingham, R. Stoothoff, D. Murdoch (eds.) The Philosophical Writings of Descartes, Cambridge: Cambridge University Press 1985 (referred as 'CSM').
[According to Descartes, existence is a property]
"Here I do not see what sort of thing you want existence to be, nor why it cannot be said to be a property just like omnipotence - provided, of course, that we take the word 'property' to stand for any attribute, or for whatever can be predicated of a thing; and this is exactly how it should be taken in this context." (AT VII, 382; CSM II 262)"
From: René Descartes, Replies to the Objections [by Gassendi] to the Fifth Meditation.
"I call a thing impossible whose nature implies that it would be contradictory for it to exist; necessary whose nature implies that it would be contradictory for it not to exist; and possible whose existence be contradictory for it to exist; necessary whose nature implies that it would be contradictory for it not to exist; and possible whose existence, (*) by its very nature, does not imply a contradiction -- either for it to exist or for it not to exist -- but whose necessity or impossibility of existence depends on causes unknown to us, so long as we feign its existence. So if its necessity or impossibility, which depends on external causes, were known to us, we would have been able to feign nothing concerning it.
 From this it follows that, if there is a God, or something omniscient, he can feign nothing at all. For as far as We are concerned, after I know that I exist, (s) I cannot feign either that I exist or that I do not exist; nor can I feign an elephant which passes through the eye of a needle; nor, after I know the nature of God, can I feign either that he exists or that he does not exist. (t) The same must be understood of the Chimera, whose nature implies that it would be contradictory for it to exist. From this what I have said is evident: that the fiction of which we are speaking here does not occur concerning eternal truths. (u) I shall also show immediately that no fiction is concerned with eternal truths.
 But before proceeding further, I must note here in passing that the same difference that exists between the essence of one thing and the essence of another also exists between the actuality or existence of the one thing and the actuality or existence of the other. So if we wished to conceive the existence of Adam, for example, through existence in general, it would be the same as if, to conceive his essence, we attended to the nature of being, so that in the end we defined him by saying that Adam is a being. Therefore, the more generally existence is conceived, the more confusedly also it is conceived, and the more easily it can be ascribed fictitiously to anything. Conversely, the more particularly it is conceived, then the more clearly it is understood, and the more difficult it is for us, [even] when we do not attend to the order of Nature, to ascribe it fictitiously to anything other than the thing itself This is worth noting." (pp. 24-25)  Now we must consider those things that are commonly said to be feigned, although we understood clearly that the thing is not really as we feign it. E.g., although I know that the earth is round, nothing prevents me from saying to someone that the earth is a hemisphere and like half an orange on a plate, or that the sun moves around the earth, and the like. 1f we attend to these things, we shall see nothing that is not compatible with what we have already said, provided we note first that we have sometimes been able to err, and now are conscious of our errors; and then, we can feign, or at least allow, that other men are in the same error, or can fall into it, as we did previously.
We can feign this, I say, so long as we see no impossibility and no necessity. Therefore, when I say to someone that the earth is not round, etc., I am doing nothing but recalling the error which I, perhaps, made, or into which I could have fallen, and afterwards feigning, or allowing, that he to whom I say this is still in the same error, or can fall into it. As I have said, I feign this so long as I see no impossibility and no necessity.
(s) Because the thing makes itself evident, provided it is understood, we require only an example, without other proof. The same is true of its contradictory -- it need only be examined for its falsity to be clear. This will be plain immediately, when we speak of fictions concerning essence.
(t) Note. Although many say that they doubt whether God exists, nevertheless they have nothing but the name, or they feign something which they call God; this does not agree with the nature of God, as I shall show later in the proper place.
(u) By an eternal truth I mean one, which, if it is affirmative, will never be able to be negative. Thus it is a first and eternal truth that God is; but that Adam thinks is not an eternal truth. That there is no Chimera is an eternal truth; but not that Adam does not think." (II, 20)"
(*) Joachim (Spinoza's Tractatus de Intellectus Emendatione, Oxford: Clarendon Press, 1940) suggests reading essentia, though the Opera Posthuma's 'existentia' is supported by the Nagelate Schriften. If it were not for the immediately following phrase (ipsa sua natura), I would think this almost certainly correct. 1 have translated the Latin as it stands, but (with Eisenberg) I feel certain that what Spinoza means is that the essence of the thing by itself does not entail either that the thing cannot, or that it must, exist. [Note by Edward Curley]
From: Treatise on the Emendation of the intellect, in: The Collected Works of Spinoza, edited and translated by Edwin Curley, Princeton: Princeton University Press 1985.
[According to Leibniz, existence is not a property]
"Existence. It can be doubted very much whether existence is a perfection or degree of reality; for it can be doubted whether existence is one of those things that can be conceived -- that is, one of the parts of essence; or whether it is only a certain imaginary concept, such as that of heat and cold, which is a denomination only of our perception, not of the nature of things. Yet if we consider more accurately, [we shall see] that we conceive something more when we think that a thing A exists, than when we think that it is possible. Therefore it seems to be true that existence is a certain degree of reality; or certainly that it is some relation to degrees of reality. Existence is not a degree of reality, however; for of every degree of reality it is possible to understand the existence as well as the possibility. Existence will therefore be the superiority of the degrees of reality of one thing over the degrees of reality of an opposed thing. That is, that which is more perfect than all things mutually incompatibles exists, and conversely what exists is more perfect than the non-existent, but it is not true that existence itself is a perfection, since it is only a certain comparative relation [comparatio] of perfections among themselves." (p. 165)
From: Robert Merrihew Adams, Leibniz. Determinist, Theist, Idealist, Oxford: Oxford University Press 1994 (undated memorandum by Leibniz).
"Christian Wolff was a rationalistic school philosopher in the German Enlightenment. During the period between the death of Leibniz (1714) and the publication of Kant's critical writings (1780s), Wolff was perhaps the most influential philosopher in Germany.
(...) Wolff thought of philosophy as that discipline which provides reasons to explain why things exist or occur and why they are even possible. Thus, he included within philosophy a much broader range of subjects than might now be recognized as 'philosophical'. Indeed for Wolff all human knowledge consists of only three disciplines: history, mathematics and philosophy.
(...) For Wolff, the immediate objective of philosophical method is to achieve certitude by establishing an order of truths within each discipline and a system within human knowledge as a whole. The ultimate goal is to establish a reliable foundation for the conduct of human affairs and the enlargement of knowledge." (pp. 776-777)
From: Charles A. Corr, "Wolff, Christian", in: Edward Craig (ed.), The Routledge Encyclopedia of Philosophy, New York: Routledge 1998.
"Wolff's Ontology begins ( 27) with the assertion of the two laws of contradiction and sufficient reason, both fundamental to the assertion that something is, or that it is not. The former requires that what is must be free from inner conflict, the latter that, if it does not, like a necessary being, have a reason for being in its own nature, it must depend on such a reason in something other than itself. The law of causation, as we ordinarily understand it, is for Wolff only a special form of the law of sufficient reason, pertinent to temporal, changeable things and their states ( 71). From these principles Wolff proceeds to the consideration of the metaphysical modalities, of which the most fundamental is the possible, the negation of the self-contradictory, or logically impossible. Everything actual, he holds, is by the law of contradiction possible, but he here embraces some invalid theorems, for instance, that a possible consequence can only have possible premisses. Obviously, modal logic is still insecure, though Wolff's treatment of apagogic proof in 98 is of some interest. From Wolffian principles it follows that the notion of an entity not wholly determinate is "imaginary', and that the indeterminate is only what is for us determinable, and that it will have to be determined by a sufficient reason ( 111, 117). There is no room in Wolffianism, any more than in Leibnizianism, for radical alternativity: Kant, however, will diverge from this position under the influence of Crusius.
All this leads, however, to Wolff's treatment of what he calls an entity: an entity is defined as any thing which can exist, to which existence is not repugnant. Thus warmth in this stone is a something, an entity, since a stone certainly can be warm or a warm stone can exist. There does not need to be any actual stone-warmth for us to have an entity before us. An entity is, however, rightly called fictitious or imaginary, if it lacks existence, which does not, however, make it less of an entity. These near-Meinongian positions are of great contemporary interest, and form the spring-board for much of Kant's later criticisms of the ontological proof, which is Wolffian enough to treat 100 possible dollars as if they certainly were something. Wolff goes on to draw the distinctions of essential features and attributes, on the one hand, which always must belong to an entity, and its modes, on the other hand, which are merely the characters that it can have and also can not have. Obviously, however, something must be added to possibility to raise it to full existence, and this Wolff is simply content to call the possibility-complement ( 174)." (pp. 39-40).
From: John N. Findlay, Kant and the Transcendental Object. A Hermeneutic Study, Oxford: Clarendon Press, 1981.
"Being is what can exist and, consequently, that with which existence is not incompatible" (§ 134)
"Hinc existentiam definio per complementum possibilitatis" (Existence is defined as the complement of possibility)." (§ 174)
From: Christian Wolff, Philosophia prima sive Ontologia (1730), (First Philosophy, or Ontology).
"Crusius was a pivotal figure in the middle period of the German Enlightenment, linking Pufendorf and Thomasius with Kant. Though sometimes wrongly characterized (for example by Hegel) as a Wolffian, he was instead an important critic of that position. His system reflected a new alliance between Pietism and Lutheran orthodoxy, offering a comprehensive antirationalist, realist, and voluntarist alternative to the neoscholastic tradition as renovated by Leibniz. Crusius was important in Kant's development and helps us understand the latter's philosophical Protestantism." (p. 736)
From: Michael J. Seidler, "Crusius, Christian August", in: Edward Craig (ed.), The Routledge Encyclopedia of Philosophy, New York: Routledge 1998.
"Crusius' influence on Kant. Recent historical scholarship has stressed Crusius' importance in Kant's development, and the view that Kant's philosophy was rooted in Wolff's system has been more and more questioned. Recent research has shown that Kant, educated in the Pietistic, eclectic, and anti-Wolffian milieu of Königsberg University, was mainly trying in his precritical development (1745-1768) -- despite the nonorthodox Wolffian influence of his teacher, Martin Knutzen -- to counteract Wolffian philosophy in an increasingly original way. He therefore appealed both to recent anti-Wolffian trends -- to Maupertuis and his Berlin circle and through Maupertuis to Newton -- and to Crusius, the new leader of Pietist philosophy and only nine years his senior, whose reputation grew tremendously from 1744 on. Crusius' influence on Kant consists in six main points, some of which were also held by other Pietist philosophers or by Maupertuis. Crusius stressed the limits of human understanding, a theme that recurs in Kant's writings under different forms from 1755 on. He rejected the Ontological Argument, as did Kant after 1755, and he later rejected all theoretical proofs of God's existence. He assumed a multiplicity of independent first principles; Kant did so after 1755. He denied the importance of formal logic, and simplified it. He rejected the possibility of defining existence, and accepted a multiplicity of simple notions. He rejected the mathematical method as applied to philosophy. Kant adopted these last three positions in 1762.
Kant's Crusianism reached its climax in his Untersuchung über die Deutlichkeit der Grundsätze der naturlichen Theologie und der Moral ("Investigations Concerning the Distinctness of the Fundamental Principles of Natural Theology and Morals," Berlin, 1764), written in 1762. By 1763 Kant's enthusiasm for Crusius' philosophy was waning, but he did not reject the six tenets above and was still influenced by Crusius on individual points as late as the 1770s. Bohatec has claimed that Crusius' doctrines in revealed theology exerted some influence on Kant's late works in religion." (p. 270).
From: Paul Edwards (ed.), Encyclopedia of Philosophy, New York: Macmillan 1967, entry "Crusius, Christian August", by Giorgio Tonelli.
[First definition of existence as a predicate]
Existence is "that predicate of a thing by virtue of which the thing is to be encountered somewhere and at some time outside thought." (section 46)
From: Christian August Crusius, Entwurf der notwendingen Venunftwharheiten, (1745), (Sketch of Necessary Truths of Reason).
"Being is that which belongs to every conceivable term, to every possible object, of thought-in short to everything that can possibly occur in any proposition, true or false, and to all such propositions themselves. Being belongs to whatever can be counted. If A be any term that can be counted as one, it is plain that A is something, and therefore that A is. 'A is not' must always be either false or meaningless. For if A were nothing, it could not be said not to be ; 'A is not' implies that there is a term A whose being is denied, and hence that A is. Thus unless 'A is not' be an empty sound, it must be false. Whatever A may be, it certainly is. Numbers, the Homeric gods, relations, chimeras and four-dimensional spaces all have being, for if they were not entities of a kind, we could make no propositions about them. Thus being is a general attribute of everything, and to mention anything is to show that it is.
Existence, on the contrary, is the prerogative of some only amongst beings. To exist is to have a specific relation to existence-a relation, by the way, which existence itself does not have. This shows, incidentally, the weakness of the existential theory of judgment-the theory, that is, that every proposition is concerned with something that exist, For if this theory were true, it would still be true that existence itself is an entity, and it must be admitted that existence does not exist, Thus the consideration of existence itself leads to non-existential pro, positions, and so contradicts the theory. The theory seems, in fact, to have arisen from neglect of the distinction between existence and being Yet this distinction is essential, if we are ever to deny the existence of anything. For what does not exist must be something, or it would be meaningless to deny its existence ; and hence we need the concept of being, as that which belongs even to the non-existent." (pp. 449-450)
From: Bertrand Russell, The Principles of Mathematics, New York: W. W. Norton Company 1903, second edition 1937.
On the website "Theory and History of Ontology" (www.ontology.co)
Nelson Michael, "Existence", The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.)