First Principles in Aristotle's Parts of Animals
Allan Gotthelf
In Gotthelf and Lennox, eds., Philosophical Issues in Aristotle's Biology
Cambridge, 1987

"The application of hypothetical necessity to the explanation of the parts of animals thus depends both on the existence of formal natures and on their acting for the sake of something, i.e. for the best from among what is possible to things of that formal nature." (GOTTHELF-1987, p. 188)

"Explanations of parts must thus (dio, 640a33) begin not from the material natures of the elements which constitute these parts, and the animals of which they are parts, but from the natures of the animal forms themselves: 'Hence we should if possible say that because this is what it is to be a man, therefore he has these things; for he cannot be without these parts. Failing that, we should get as near as possible to it: we should say altogether that it cannot be otherwise, or that it is at least as good thus.'" (GOTTHELF-1987, pp. 188-189)

END