I have already, with much frustration, attempted to make sense of the beginning of Lamda 3.  In light of the rest of the remaining parts of the chapter, it strikes me that there is a possibility that Aristotle had some Third Man-ish worries in mind.  But, be that as it may or may not be, there is undeniably an assault on the Theory of the Forms here. 

In conjunction with his opening statement that there must be 1) a subject 2) a form 3) an initial mover, Aristotle says we must next,

“observe that every substance is generated from something which has the same name (“substances” including not only natural but all other products). Things are generated either by art or by nature or by chance or spontaneously. Art is a generative principle in something else; nature is a generative principle in the subject itself (for man begets man); the other causes are privations of these” (1070a4-10 trans. Treddenick) (1).

Aristotle continues on with an explication of form, matter and the hylomorphic compound consisting of form and matter.  Then he notes,

“Moving causes are causes in the sense of preexistent things, but formal causes coexist with their effects. For it is when the man becomes healthy that health exists, and the shape of the bronze sphere comes into being simultaneously with the bronze sphere”  (1070a21-24 trans. Treddenick) (2).

He concludes by referencing his aphoristic “man begets man” from earlier in the chapter and says,

“Clearly then there is no need on these grounds for the Ideas to exist; for man begets man, the individual begetting the particular person. And the same is true of the arts, for the art of medicine is the formula of health”  (1070a26-30 trans. Treddenick) (3).

Aristotle it appears, is using nature, form, and account interchangeably in this chapter, for at least one important reason: to emphasize that form is a “generative principle in the subject itself,” and does not arise from outside itself.  This claim is already at odds with Plato’s concept of Form, and “man begets man” is evidently a common sense vindication of how things come to be, from which there is no deviation.  Indeed figs don’t come from thistles.

Plato dismisses Aristotle’s dictum that, “formal causes coexist with their effects,” i.e., they come to be at the same time (ἅμα).  Plato instead maintains that forms precede their effects, and that, returning us to the beginning of the chapter, conflates the form and the initial mover.  Thus, instead of  1) a subject 2) a form 3) an initial mover, there is 1) a subject and 2) a form/initial mover.  If a form is an external force, as it would have to be on Plato’s view, then by Aristotle’s understanding above (“Art is a generative principle in something else”) this would make a form an art.  But Plato’s concept of form is clearly not an art, so either form has some other definition or there are no forms.

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Notes:

(1)
Μετὰ ταῦτα ὅτι ἑκάστη ἐκ συνωνύμου γίγνεται οὐσία· τὰ γὰρ φύσει οὐσίαι καὶ τἆλλα· ἢ γὰρ τέχνῃ ἢ φύσει γίγνεται ἢ τύχῃ ἢ τῷ αὐτομάτῳ. ἡ μὲν οὖν τέχνη ἀρχὴ ἐν ἄλλῳ, ἡ δὲ φύσις ἀρχὴ ἐν αὐτῷ (ἄνθρωπος γὰρ ἄνθρωπον γεννᾷ), αἱ δὲ λοιπαὶ αἰτίαι στερήσεις τούτων.

(2)
Τὰ μὲν οὖν κινοῦντα αἴτια ὡς προγεγενημένα ὄντα, τὰ δ᾿ ὡς ὁ λόγος ἅμα. ὅτε γὰρ ὑγιαίνει ὁ ἄνθρωπος, τότε καὶ ἡ ὑγίεια ἔστιν, καὶ τὸ σχῆμα τῆς χαλκῆς σφαίρας ἅμα καὶ ἡ χαλκῆ σφαῖρα.

(3)
φανερὸν δὴ ὅτι οὐδὲν δεῖ διά γε ταῦτ᾿ εἶναι τὰς ἰδέας· ἄνθρωπος γὰρ ἄνθρωπον γεννᾷ, ὁ καθ᾿ ἕκαστον τὸν τινά. ὁμοίως δὲ καὶ ἐπὶ τῶν τεχνῶν· ἡ γὰρ ἰατρικὴ τέχνη ὁ λόγος τῆς ὑγιείας ἐστί.