τὸ τί ἦν εἶναι is an odd phrase, common to Aristotelian diction, used when the philosopher wishes to speak about the essence of a particular thing. Most students translate the phrase as “essence” by rote, because they have not the faintest conception on how to penetrate the meaning of this four-word hieroglyphic.
Let us begin by discussing what this construction consists of at its most basic level. Fundamentally the phrase is an articular infinitive. Dr. Smyth tells us that, “The articular infinitive, while having the character of a substantive, retains the functions of a verb” (See Smyth, 2025 and following). The “character” of a substantive means that we are able to decline the infinitive as a neuter singular noun, if we place the appropriately declined definite article (τό, τοῦ, τῷ, τό) in front of it. Thus, τὸ ποιεῖν can be translated not merely as “to make,” but also as “making.” With this in mind, τὸ εἶναι, is “to be” or “being,” often simplified by most translators to “essence.”
This leaves us with the two inner terms, τί ἦν. First let us look at the imperfect ἦν. In Smyth 1901-1902 we are told that the imperfect can be used for the present tense. Liddell and Scott (εἰμί F. bottom of entry) inform us that ἦν is sometimes used as the present, corroborating the account given by Smyth. Liddell and Scott also make mention of Aristotle’s exact phrase, remarking that, “τὸ τί ἦν εἶναι expresses the essential nature of a thing.” Thus the ἦν is actually an ἐστί, at least for translation purposes.
The LSJ entry is further helpful in determining the meaning of τί ἦν as a two-word phrase. It points out that τί ἦν, in the phrase τὸ τί ἦν εἶναι, takes the place of a very similar articular infinitive, but with a dative phrase, such as τὸ ἀγαθῷ εἶναι, which can be seen in Prior Analytics 67b12 and De Anima 429b10. τί ἦν is therefore really (τῷ) τί ἦν. τί, of course, is the interrogative pronoun, “what.” The phrase τί ἦν means, “what is it?” or as an indirect interrogative, which it could also be, “what it is.”
Putting it all together in a different order we have, τὸ εἶναι “being,” (τῷ) τί ἦν “for what is it?” or as an indirect interrogative, “for what it is.” Very often when there is a dative with a verb like εἰμί, it is construed as a dative of possession, which can be translated as a genitive. We could translate τὸ τί ἦν εἶναι as, “The essence/being of what it is.” The mystery of the phrase is solved. We are nevertheless saddled with an uncharacteristically unwieldy phrase to describe a common Greek philosophical term.