 As Aristotle posits, notably in the first book of the Metaphysics, searching for causes defines philosophical inquiry; knowing the cause of x allows you to understand what x consists in. And, crucially in Aristotle’s eyes, the final cause is what matters the most: when one knows the final cause of x, one can truly understand not only what x consists in, but should consist in if it is to be the x it is supposed to be. More precisely, the end of x amounts to the ergon, literally, the “work” or the “activity”, or what we more usually call the “function” that x performs; thus, for an eye, its aim or end amounts to its function, or “functioning” (or “working”, energeia) which is, of course, its seeing. Similarly, in the case of a hand: its function is grasping things, and when we talk of the hand of a dead body, we use the name “hand” only homonymously —a dead, non-functioning hand is no longer what it is to be a hand, or a “real” hand. And when an eye is seeing well, this is what Aristotle calls its entelecheia, that is when it performs its telos in a perfect (entelês) way. It is true that in the Poetics, we don’t find the typically Aristotelian technical words energeia and entelecheia. But presumably one may take the phrase “the best tragedy” (kallistê tragôgia) which is emphatically used in Poetics 13 as the equivalent, in common parlance, to a tragedy in entelecheia, that is a tragedy that performs its function, or its telos, in the best possible way. This phrase does not refer to any particular outstanding tragedy (say, Sophocles’ Oedipus-Rex, which Aristotle seems to like very much), or to an ideal tragedy that a poet should try to emulate, but rather to any tragedy that would or does indeed fulfill its function properly.