At Nicomachean Ethics 1096a23 Aristotle resumes a list of short and precisely aimed critiques at Platonic Forms, incorporating his own theory of Categories in order to undermine a very important doctrine, the unity of Forms.  Aristotle makes two accusations at this point, both attacking this supposed unity of the Forms:

And again since the good is said in as many ways as being (for something is said in the category of what, such as god and intellect, and in quality, as the virtues, and in how much, as the moderate, and in relation, such as the useful, and in time such as the appropriate moment, in place such as dwelling, and other such examples) it is clear that there would not be some universal (καθόλου) that is both common (κοινόν) and singular (ἕν).  For it is is said not in all categories, but in one alone.  And again since there is also one science of things in regards to one Form, there would also be some one science of all the goods.  But as it is, there are now many sciences even coming under a single category, such as time, for example the general’s [use of time] time in war and the doctor’s [use of time] in sickness and in regards to moderation there is medicine in regards to a diet and exercise in regards to exertion (Nicomachean Ethics 1096a23-34, Greek Follows Below). ¹⁾ἔτι δ’ ἐπεὶ τἀγαθὸν ἰσαχῶς λέγεται τῷ ὄντι (καὶ γὰρ ἐν τῷ τί λέγεται, οἷον ὁ θεὸς καὶ ὁ νοῦς, καὶ ἐν τῷ ποιῷ αἱ ἀρεταί, καὶ ἐν τῷ ποσῷ τὸ μέ-   (25) τριον, καὶ ἐν τῷ πρός τι τὸ χρήσιμον, καὶ ἐν χρόνῳ καιρός, καὶ ἐν τόπῳ δίαιτα καὶ ἕτερα τοιαῦτα), δῆλον ὡς οὐκ ἂν εἴη κοινόν τι καθόλου καὶ ἕν· οὐ γὰρ ἂν ἐλέγετ’ ἐν πάσαις ταῖς κατηγορίαις, ἀλλ’ ἐν μιᾷ μόνῃ. ἔτι δ’ ἐπεὶ τῶν κατὰ μίαν ἰδέαν μία καὶ ἐπιστήμη, καὶ τῶν ἀγαθῶν ἁπάν-   (30) των ἦν ἂν μία τις ἐπιστήμη· νῦν δ’ εἰσὶ πολλαὶ καὶ τῶν ὑπὸ μίαν κατηγορίαν, οἷον καιροῦ, ἐν πολέμῳ μὲν γὰρ στρατηγικὴ ἐν νόσῳ δ’ ἰατρική, καὶ τοῦ μετρίου ἐν τροφῇ μὲν ἰατρικὴ ἐν πόνοις δὲ γυμναστική.

Aristotle’s first criticism amounts to saying that there is not one single use of the “good” which can apply to all the examples of things which can be offered forth as good.  We can say that god is good, but this applies to the category of what, and we can say that the moderate is good, but this is only in the category of how much.  The point here would be that we could not say that that there is one “good” which applies (in the same way) to god and the moderate and however many other ways there are to say that something is “good” in some sense.  If there is no single Form of the good that can account for multiple goods in these ways, then the Form of the good is in some queer and distinct way utterly different than any of these other ways of being good.  But if that is the case, the Form of the good could not really be an explanation or cause of these particular things being good, as Platonists would have it, at any rate, since it would be good in some different sense than they.

On a similar line of thinking, there is also the concern that if there is one science (or body of knowledge) of the forms, this same science would apply to all the goods.  Sympathizing with the Platonist for a second, we could make an analogy.  It would be like saying that there is a science of being a handyman.  To have mastery of this science would include knowledge of plumbing and the knowledge of wiring and electricity and however many other forms of knowledge are required of one adept around the house.  Thus to have knowledge of “the good” would be to possess knowledge of all forms of good.  Perhaps Aristotle brings up this particular objection because in it he sees a response to the previous objection.  That is, he sees that a Platonist might respond to the charge that good is said in many ways by saying that these all fit under the umbrella of a good that is different, in a significant sense, from any particular instantiations of it.  A serious problem ensues, however, if the Form of the good is different from any particular instances of good.  The Form of the good cannot be said to be good in the same sense as one of its supposed examples.  In such as case, there would be no instantiations of good things at all!  Now let us return to the issue of knowledge, and the analogy with the handyman.  It is obvious that to know plumbing alone is not sufficient to be a handyman.  One must also have the skills of an electrician, carpenter, etc.  So, keeping with the analogy, one cannot have knowledge of the science of one good and thus have knowledge of the Form of the good.  Likewise, one cannot have knowledge of the the Form of the good because this would be like the electrician having the knowledge of the plumber, simply in virtue of having the knowledge of an electrician.  Why would this be, you ask?  Because in order to say that the plumber, by having knowledge only of plumbing, has the knowledge of a handyman, one would have to say he has the knowledge of a handyman (i.e. electricity, carpentry, etc in addition to the knowledge of plumbing.)  However, he does not have thiknowledge, so in whatever sense we can call him a handyman, it is in a significant sense different from a real handyman.