We had perhaps better consider the universal good and discuss thoroughly what is meant by it, although such an inquiry is made an uphill one by the fact that the Forms have been introduced by friends of our own. Yet it would perhaps be thought to be better, indeed to be our duty, for the sake of maintaining the truth even to destroy what touches us closely, especially as we are philosophers or lovers of wisdom; for, while both are dear, piety requires us to honour truth above our friends.
We have been discussing what is the proper end of ethics. Aristotle points out that we also need to come to a general idea about what it means for something to be 'good,' since we surely want something good to be the end of our ethics. But what is it 'to be good'? Aristotle not-very-gently suggests that Plato and the other Platonists of the Academy, whom he does not name but who were his friends as well as his teachers, have caused a lot of difficulty by introducing their bad ideas about Forms. He feels a duty as a philosopher (which just means 'lover of wisdom': philo - sophia) to try to get at the truth even though it means refuting his friends publicly.
Aristotle and Plato both believed in what we call hylomorphism. This word means that things are composed of both matter-that-can-be-organized-lots-of-ways (hyle means 'wood,' but stands here for any sort of matter that can be used to construct something else, as wood can be made into a bed or a ship or a house) that is then organized according to a form (morphē- here is 'form'). They differ quite a bit in their conception of how this relationship works.
Aside: while we are talking about this today in order to understand Aristotle, these metaphysics are not as arcane as you might imagine. You can read a paper I wrote defending the idea that hylomorphism is actually a better way of thinking about the world we live in according to our modern scientific understanding than the usual alternatives.
For Plato, the Forms are somewhere else: exactly where is not clear from what we have left of the Academy's thought. Later Neoplatonists, from Plotinus, perhaps were carrying forward Platonist ideas in asserting that there must be One that is ultimate and unified, and in the mind of that One would be the forms as Ideas: ideas in the mind of God, if you like. Aristotle, who was directly taught by Plato, will use the words Form and Idea interchangeably in this sense, so it could be that Plato's doctrine was indeed close to that (though there seem to have been two principles for Plato: the One, and something opposed to it. Avicenna demonstrates the impossibility of this 'dyad' concept in his Metaphysics of the Healing). In any case the Form of the Lion is not in the individual lion, but is a feature of basic reality whose independent existence from any actual lions is what enables lions to exist in the world. If the idea of the lion didn't exist in the mind of God, to extend that helpful metaphor, reality would not support lions existing in the world.
Aristotle has a much more straightforward account of how this works. Form for him is 'the way in which the matter is organized,' and therefore the form is in the thing. If you have some wood and you put it into the form of a table, you have a table. If you put it into the form of a chair instead, you have a chair and not a table. The wood doesn't change: in fact, if you imagine that you can use all and exactly the same wood to make either a chair or a table, you will see the point of the doctrine. The wood is the same wood whether it is a chair, a table, or a pile of parts lying on the floor. Thus, the material is the same. What changes is the form of organization.
Importantly, though the form is in the thing -- or, as you might prefer to say, the thing is in the form -- the form is not itself material. The material, again, hasn't changed. The form is therefore immaterial; and this deduction proves that materialism, which most intellectuals believe in today, is false. The immaterial not only exists, it is fundamental to things being whatever it is that they are.
So, with all that background, we can begin trying to understand what Aristotle wants to say about 'the good.'
The men who introduced this doctrine [i.e. Plato's doctrine of Forms] did not posit Ideas of classes within which they recognized priority and posteriority (which is the reason why they did not maintain the existence of an Idea embracing all numbers); but the term 'good' is used both in the category of substance and in that of quality and in that of relation, and that which is per se, i.e. substance, is prior in nature to the relative (for the latter is like an off shoot and accident of being); so that there could not be a common Idea set over all these goods.
This sentence employs a lot of mental furniture that Aristotle himself built. Aristotle divided the world into two kinds of things: substances and attributes. Substances are things like men or horses -- those are Aristotle's favorite examples -- and they are the primary things (just as he says here). Attributes only exist as qualities that belong to substances.
"Relation" is a particular kind of attribute. It is a little clumsy, because a relationship links two things. Say you have a father and a son; there is, then, a relation attribute that links them. If attributes only exist insofar as they belong to a substance, which one does it belong to?
The answer for Aristotle is that there are two relations: the father has one of being "related to the son," and the son has a different one of being "related to the father." That's a very strange way of thinking about relationships -- and, I think, wrong: relationships are real, which is a metaphysical discussion for another day -- but it does allow Aristotle to preserve his metaphysical model of only two kinds of things existing.
So, you can now understand his objection: "good" as an idea can't be the same thing for substances as for attributes, because they differ in priority. Substances are the primary things of the world. The attributes don't matter nearly as much, and can't exist independently. What's good for the substance is good in a higher and better way than anything could be good 'for an attribute.'
Plato might reply that the Idea of the Good (or the Form of the Good) still is necessary for understanding what it means for things to be good for anything at all; the fact that there might be a 'good for substances' and 'good for attributes' doesn't mean there isn't a 'Good in itself' standing above those two subcategories and unifying them. But we aren't here to discuss Plato today.
Further, since 'good' has as many senses as 'being' (for it is predicated both in the category of substance, as of God and of reason, and in quality, i.e. of the virtues, and in quantity, i.e. of that which is moderate, and in relation, i.e. of the useful, and in time, i.e. of the right opportunity, and in place, i.e. of the right locality and the like), clearly it cannot be something universally present in all cases and single; for then it could not have been predicated in all the categories but in one only.
The categories are Aristotle's too -- we have a book by that name from him, and you will find his explanation of substance and attribute in it as well as many other things. You can see the point he is making: if Good was only one, it would belong to one category, not all the categories. Plato's rebuttal would presumably be the same: for the concept of 'goodness per se' to make any sense, there has to be an overarching unity that connects all the different goods in the various categories.
Further, since of the things answering to one Idea there is one science, there would have been one science of all the goods; but as it is there are many sciences even of the things that fall under one category, e.g. of opportunity, for opportunity in war is studied by strategics and in disease by medicine, and the moderate in food is studied by medicine and in exercise by the science of gymnastics.
This is a very similar objection. Why isn't there a Science of the Good, which contains all knowledge pertaining to the pursuit of any good thing? (In fact, there is: metaphysics is the science of existence, and existence -- we shall learn from Aristotle and those who followed him -- is the same thing as goodness. Aquinas has quite a lot to say about that point, following Aristotle and Avicenna.)
And one might ask the question, what in the world they mean by 'a thing itself', is (as is the case) in 'man himself' and in a particular man the account of man is one and the same. For in so far as they are man, they will in no respect differ; and if this is so, neither will 'good itself' and particular goods, in so far as they are good. But again it will not be good any the more for being eternal, since that which lasts long is no whiter than that which perishes in a day. The Pythagoreans seem to give a more plausible account of the good, when they place the one in the column of goods; and it is they that Speusippus seems to have followed.
