Although Plato’s and Aristotle’s moral theories are quite similar, in Book I of the Nicomachean Ethics, Aristotle criticizes Plato’s notion of the form of the good. To understand this criticism, you need to have some understanding of Plato’s doctrine of the forms.
Consider a chair.
What is it in virtue of which a chair is a chair? That is, although there are many different kinds of chairs,
if the term “chair” is to have definite meaning, there must be something in common to all chairs. This is what an artisan must have knowledge of if s/he is to fabricate a chair.
This thing that is common to all chairs – that all particular chairs “participate in”, is called “the form of the chair”, or “chairness”.
Now, imagine that because of some great catastrophe (perhaps a revolution by the anti-asseoiristes), all actual chairs in the world destroyed. But even if all actual chairs were to be destroyed, this would not mean that the form of chairness had been destroyed, for it would still be possible for a craftsman to make another chair. So, we might be tempted to say that the form of the chair exists in the mind of the craftsman. However, three things might prevent us from saying this.
1) Different craftsmen probably have slightly different ideas of chairness. Yet there must be something in virtue of which all of these ideas of chairness are still ideas of “chairness”. So, the form of chairness cannot exist only in the minds of craftsmen. The form of chairness must exist elsewhere.
2) What if all craftmen who knew about chairs were to pass away (perhaps killed in the revolution)? Would chairness then cease to exist? It would if the form of chairness existed in the mind of craftsmen only. However, would it not be possible for someone to come along years later (a counter-revolutionary!) and invent a chair? For this to be possible, the form of the chair must exist somewhere apart from any particular chair and apart from any person’s idea of a chair.
3) The trouble with chairs and individual people’s ideas of chairs is that they exist in the world of becoming. That is, specific chairs, that are made of wood, steel, naugahide (and so on) come into and pass out of existence. They can be destroyed. The same is true of the ideas of chairs that people have. Because they can be destroyed, they are less perfect than the form of the chair, which does not come into or pass out of existence. So the form of chairness cannot exist in the world of becoming. Rather, it exists in the world of pure being – a world that is more perfect than the material world. It is this world that the philosopher visits in Socrates’ allegory of the cave.
· So, for any determinate thing, there must be some form in virtue of which that thing is what it is.
· The form is separate from any particular instance of it. (The form isn’t in any particular chair.)
· The form is separate from any individual’s mental image of it.
· The form is superior to particular thing or any mental image of that thing because it is perfect and because it doesn’t deteriorate or cease to be.
Now, in addition to forms of material things (chairs, ducks, circles, human beings), there are forms of Truth, Beauty, and Goodness.
The form of Truth is the form that all things that are true participate in. The Form of the True is that in virtue of which all true things are true. The form of truth is separate from any true thing.
The form of Beauty is the form that all beautiful things (flowers, vistas, beautiful people) participate in. The form of the True is that in virtue of which all beautiful things are beautiful. The Form of Beauty is separate from any beautiful thing.
The form of Good is the form that all good things participate in. The form of the Good is that in virtue of which all good things are good. The Form of the Good is that in virtue of which all good things are good.
Now, since both Truth and Beauty are Good things, they both participate in the Form of the Good. Thus, the form of the Good is separate from and superior to the forms of Truth and Beauty.
This is a quick and dirty version of Plato’s doctrine of the forms. If we want to know about goodness or how to be good or what acts are good acts, according to Plato, what we must study is the Form of the Good.
So, Plato held that forms are separate (from particulars) and eternal. He also held that the highest good must be eternal, separate from the physical world in which things come to be and pass away. (Because something that lasts is better than something that doesn’t.)
(There’s a lot more to Plato’s doctrine of the forms, but this should be enough for you to begin Aristotle’s criticism. If you want to know more, check out the Platonic dialogues, e.g., The Republic.)
Aristotle criticizes Plato’s doctrine of the forms.