Perhaps Avicenna held this view as well.^23 I am also inclined to attribute the view to
Leibniz circa the period in which he accepted the complete concept theory of
substance, according to which what it is to be a substance is just to have a concept
so complete that all truths about this substance are contained in this concept.^24
During this period, Leibniz happily distinguishes the actual Adam, whose complete
concept is actually exemplified, from merely possible Adams, each of whom has
distinct complete concepts, none of which is actually exemplified.^25 Leibniz does not,
however, appear to traffic in complete concepts of impossible objects. Complete
concepts are, for Leibniz, good candidates for being the essences of substances.^26
And there are several places in Leibniz’s work where the connection between real
definition and possibility is explicitly asserted.^27
I distinguish the Cartesian view (about strict essence) from the distinct epistemo-
logical claim that, in principle, we can know some strict essentialist claims aboutx
prior to knowing whetherxin fact exists.^28 That said, the Cartesian view about strict
Descartes (1991a: 354) tells us that knowing the essence of something doesn’t entail that the thing exists.
See also Descartes (1991b: 343; 1992: 197). See also Curley (1978: 147–50) and Secada (2000: 59–60), who
also attribute the Cartesian view to Descartes. For a recent proponent of something like this view, see
Tahko (2013: 53), who claims that only possible existents have strict essences, and that even merely non-
existent possible entities (2013: 56) have essences too. Correia (2006: 764) holds that essential truths are
necessary truths, which suggests that he rejects the existentialist view discussed below; but it is not clear
which of the other views discussed here he might favor. Trogdon (2013: 467) also accepts that essential
truths are necessary truths.
(^23) For discussion of Avicenna, see Wisnovsky (2003: 10–11, 158–60; 2005: 105–13) and Druart (2005:
337 – 8). Miller (2002: 14) claims that Avicenna holds that an essence that could possibly be exemplified can
be contemplated by a mind even if no individual has that essence, and hence existence is“detachable”from
essence; he also claims that Avicenna recognizes a mode of being particular to essences,esse essentiae.
Kenny (2005a: 45–6) suggests something similar, but note that Kenny (2005a: 88) also suggests that
Avicenna believes in merely possible individuals, which suggests that he attributes the possibilist view to
Avicenna as well. 24
25 This is defended in hisDiscourse on Metaphysics, included in Leibniz (1989).
See Leibniz (1989: 72–3). Leibniz endorsed a kind of possibilism prior to theDiscourse on Metaphys-
icsin which there are possible things that do not exist that nonetheless have“definitions,”but in order to
discern the reason why an object exists we must go beyond its“definition”; see Leibniz (1989: 19–20). In a
later work, Leibniz (1989: 114–15) says that the essences of created things are eternal even though the
created things are not, and that these essences depend on God 26 ’s understanding but not his will.
Adams (1994: 14) sounds a note of caution; he suggests that even in theDiscourse on Metaphysics,in
which Leibniz appears to identify complete concepts with essences, it might be better to think of the
essence of a substance as that which in conjunction with the essences of all other substances generates the
complete concept of that substance; see also Adams (1994: 58). That said, even on this interpretation, a
merely possible Adam has a complete concept, and so must have an essence. 27
See, for example, hisPrimary Truths, in which Leibniz (1989: 26) asserts that a real definition of a
thing provides a proof of the possibility of that thing; Leibniz (1989: 273) asserts that the essences of things
are co-eternal with God but their existences are not. See also Adams (1994: 136–9) for relevant discussion.
The Cartesian view is also endorsed by Leibniz’s successors, such as Christian Wolff, who holds both that
the essence of a thing is necessary and immutable, but also that having an essence implies being a possible
existent. See Watkins (2009: 13 28 – 14) for the relevant passages from Wolff.
Descartes (1991a: 78) tells us that, according to the true logic, we must never ask of the existence of a
thing withoutfirst knowing its essence. Tahko (2013: 60–3) presents the search for the Higgs boson as a
case in which an essence is known prior to the existence of the thing. (My inclination is to deny that we
have grasped an essence in that case, but merely have grasped a theoretical role that an entity could play. In