Stony Brook University

Another example of the difference is found in Leibniz’s “Meditationes de
Cognitione, Veritate, et Ideis” (1684).
And so we also have a distinction between nominal definitions, which
contain only marks of a thing to be distinguished from other things, and
real definitions, from which one establishes that a thing is possible. And
with this we give our due to Hobbes, who claimed that truths are arbitrary,
since they depend on nominal definitions, without considering the fact that
the reality of a definition is not a matter of decision and that not just any
notions can be joined to one another. Nominal definitions are insufficient
for perfect knowledge except when one establishes in another way that the
thing defined is possible. It is also obvious, at last, what true and false
ideas are; namely, an idea is true when its notion is possible and false
when it includes a contradiction. (AG 26) 48
When Leibniz speaks of “marks” he usually means some empirical characteristic of a
thing. But the notion of ‘possibility’ is the true key to Leibniz’s realism about definition.
He goes on to say, “the possibility of a thing is known a priori when we resolve a notion
into its requisites, that is, into other notions known to be possible, and we know that there
is nothing incompatible among them” (AG 26). 49 By analyzing definitions, and by
applying the principles of identity and non-contradiction, we can be certain that the
definition contains no incompatible ideas. If it does not, then we say that the definition is
possible. This way, Couturat says, Leibniz can, unlike Hobbes, maintain that “a real
definition is not arbitrary like the simple imposition of a name, because it corresponds to
a true “essence,” to a possible “nature” which does not depend on our good pleasure” (C
190). Regarding essence, Leibniz says in the Nouveaux Essais, “Essence is fundamentally
nothing but the possibility of the thing under consideration; something which is thought
possible is expressed by a definition; but if this definition does not at the same time
express this possibility then it is merely nominal” (NE 293). 50 In other words, nominal
definitions are potentially confused and contradictory, whereas real definitions reveal the
possibility of a thing. The possibility of a thing is determined when it is shown (i.e., when
it is demonstrated) that there is no confusion or contradiction among its notions.
Experience, however, reveals the thing’s actuality (NE 294). Furthermore, Leibniz holds
that “a thing has only one essence;” however, “this can be expressed by many
definitions,” just as a town can be represented by different drawings” (NE 294). Still, the
48 A.6.4.589: “Atque ita habemus quoque discrimen inter definitiones nominales, quae notas tantum rei ab
aliis discernendae continent, et reales, ex quibus constat rem esse possibilem, et hac ratione satisfit Hobbio,
qui veritates volebat esse arbitrarias, quia ex definitionibus nominalibus penderent, non considerans,
realitatem definitionis in arbitrio non esse, nec quaslibet notiones inter se posse conjungi. Nec definitiones
nominales sufficiunt ad perfectam scientiam, nisi quando aliunde constat rem definitam esse possibilem.
Patet etiam, quae tandem sit Idea vera, quae falsa, vera scilicet cum notio est possibilis, falsa cum
contradictionem involvit.”
49 A.6.4.589: “Possibilitatem autem rei vel a priori cognoscimus, vel a posteriori. Et quidem a priori, cum
notionem resolvimus in sua requisita, seu in alias notiones cognitae possibilitatis, nihilque in illis
incompatibile esse scimus.”
50 A.6.6.293: “L’essence dans le fonds n’est autre chose que la possibilité de ce qu’on propose. Ce qu’on
suppose possible est exprimé par la definition, mais cette definition n’est que nominale quand elle
n’exprime point en même temps la possibilité.”
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