Aristotle turns in Ζ.4 to a consideration of the next candidate for substance: essence. (‘Essence’ is the standard English translation of Aristotle's curious phrase
to ti ên einai, literally “the what it was to be” for a thing. This phrase so boggled his Roman translators that they coined the word
essentia to render the entire phrase, and it is from this Latin word that ours derives. Aristotle also sometimes uses the shorter phrase
to ti esti, literally “the what it is,” for approximately the same idea.) In his
logical works, Aristotle links the notion of essence to that of definition (
horismos) — “a definition is an account (
logos) that signifies an essence” (
Topics 102a3) — and he links both of these notions to a certain kind of
per se predication (
kath’ hauto, literally, “in respect of itself”
— “what belongs to a thing in respect of itself belongs to it in its essence (
en tôi ti esti)” for we refer to it “in the account that states the essence” (
Posterior Analytics, 73a34-5). He reiterates these ideas in Ζ.4: “there is an essence of just those things whose
logos is a definition” (1030a6), “the essence of a thing is what it is said to be in respect of itself” (1029b14). It is important to remember that for Aristotle, one defines things, not words. The definition of tiger does not tell us the meaning of the word ‘tiger’; it tells us what it is to be a tiger, what a tiger is said to be in respect of itself. Thus, the definition of tiger states the essence — the “what it is to be” of a tiger, what is predicated of the tiger
per se.