“If we go even further back and examine the oldest stages of the Indo-European language, it emerges that really the main opposition is between active and middle and that the passive voice is something additional that grew up and developed later. In Greek this can still be seen especially in the fact that there are basically no specifically passive forms at all: passive functions are served partly by the same forms as the middle and partly by the specially deployed active forms, the former in the present and the perfect, the latter in the aorist. Now, as soon as we regard active and middle as fundamental and primary, the so-called deponents become clear, of which Greek has a great number, as well as Latin. Our account will now be as follows: there are (1) verbs with both active and middle endings, whose middle forms can sometimes have passive meaning, e.g. φέρω (‘I carry’), φέρομαι (‘I carry for myself’ or ‘I win’ (middle), or (passive) ‘I am carried’); (2) verbs which occur only in active forms, such as κλύω, στείχω, στίλβω, φεύω (‘hear’, ‘go’, ‘gleam’, ‘flee’); and (3) verbs which occur only in middle forms, such as ἧμαι, κεῖμαι, νέομαι (‘I sit’, ‘I lie’, ‘I go, come’), and there are more of type 3 than of type 2 (active only). In other words, deponents are simply middle verbs which have no active forms and our task becomes to discover the middle meaning in the deponents.”
—Jacob Wackernagel (trans. David Langslow), Lectures on syntax: With special reference to Greek, Latin, and Germanic, 160 (vol 1, 121 in the German edition). Bold text is original (at least to Langslow).
Even 100 years ago (the original German edition was published between 1920 and 1924), linguists studying Greek knew that “deponent” was a misnomer. We’ve just been slow to keep up. Still, we’ve gone farther and farther in the efforts to tracking down and explaining middle semantics today in a way that was not done systematically in the last century. But that’s the main benefit of contemporary linguistic science.
“[D]eponents are simply middle verbs which have no active forms and our task becomes to discover the middle meaning in the deponents”