By the time we are nearly to the end of Metaphysics Lambda 3, Aristotle, perhaps surprisingly, announces that the preceding has been in some way a rebuke of Platonic Forms.  Aristotle says,

Hence Plato was not far wrong in saying that there are as many Forms as there are kinds of natural objects; that is if there are Forms distinct from the things of our world (Metaphysics 1070a18-19, trans. Hugh Tredennick). (1) 

Along with other implications I take this to mean that, if there are even such (implausible) entities as Plato’s Forms, the most likely candidates for them would be Forms of natural objects, not Forms of Pez dispensers and shoeboxes.

One important observation to take away from this, which we will return to, is that Aristotle takes Plato to be asserting that Forms are causes of substances in some sense.  This is shown by his following remark that asserts a distinction between moving and formal causes, a sentence which translators often choose erroneously to set off as a new paragraph.

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. Tredennick). (2)

Aristotle here is saying something about the causal nature of forms, whether his or Plato’s, namely that they are coterminous with their effects.  I take this to be a logical and not merely chronological relation.  More generally, however, Aristotle has already introduced the notion of how different substances come to be,

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 [i.e. chance and spontaneity] are privations of these (1070a4-a9 trans. Tredennick). (3)

So Aristotle has made the point that forms come about at the same time (ἅμα) as their effects, and here he presumably sets out four causes of substances that are exhaustive, and mutually exclusive.

Of these four causes of art, nature, chance, spontaneity, I take it that Plato, wishing to maintain that Forms are a cause of substance would deny that they bring about substance by chance or spontaneity.  This leaves nature and art as the kinds of causes for Forms.  And as can be gleaned from all of chapter 3, Plato wants to uphold the idea that Forms are transcendent, or to put it less loftily, they are not immanent within their substance, as Aristotle would have it.  Yet this would preclude Forms from being a natural cause, for “nature is a generative principle in the subject itself (for man begets man).”  This, of course, would play into Aristotle’s preferred definition of form.  This leaves art alone as the candidate cause for Forms.  I do not see an argument here directly addressed to this possibilty, but Aristotle does say that, “In some cases the individuality does not exist apart from the composite substance (e.g., the form of a house does not exist separately, except as the art of building” (1070a13-15 trans. Tredennick) (4).  This would seem to grant the desire to Plato to have his Forms aloof from the present world, but at a cost of making them merely instrumental, and worse perhaps, dependent on something else to initiate the causality, a craftsman.  This possibility, however, was dissolved when Aristotle said that a formal cause operates simultaneously with its effect, therefore a form cannot be an art.  

REFERENCES:

(1) διὸ δὴ οὐ κακῶς Πλάτων ἔφη ὅτι εἴδη ἐστὶν ὁπόσα φύσει, εἴπερ ἔστιν εἴδη ἄλλα 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) 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.

(4) ἐπὶ μὲν οὖν τινῶν τὸ τόδε τι οὐκ ἔστι παρὰ τὴν συνθέτην οὐσίαν (οἷον οἰκίας τὸ εἶδος, εἰ μὴ ἡ τέχνη·