How does Aristotle motivate that there is no universal good?

Question

I'm reading the Nicomachean Ethics written by Aristotle and there's this one statement (and it's motivations) that I'm really struggling with. In my version of the book it is written in rather proficient English and I'm having a hard time understanding what it says.

So, in chapter I (The Object of Life) under the 6th statement ("There cannot be a universal good such as Plato held to be in his theory of forms"), Aristotle writes this:

But things are called good both in the category of substance and in that of quality and in that of relation; and what exists in its own right, namely substance, is by nature prior to what is relative (for this is a sort of offshoot of attribute of that which exists); so that there cannot be any common idea in these cases.

I have read this over and over again and I can still not understand it. I don't have any knowledge of Plato's Theory of Forms, so that might be the reason, if Aristotle is referring to that. Aristotle then says:

Again, things are called good in as many senses as they are said to exists; for they are so called in the category of substance (e.g. god or mind) and in quality (the virtues) and in quantity (what is moderate) and in relation (what is useful) and in time (opportunity) and in place (habitat and so on).

This I understood better and it seems rather obvious, but I still cannot make up what the first sentence would mean.

What is that Aristotle means in the first sentence and how does he motivate that there is no universal good?

Answer 1

Plato held that the Form of the Good was the Form over all the others. It would function as the Form of Forms, then, giving them their being and intelligibility. On Aristotle's view, then, if there is no category over the other categories, then if a universal standard of good ranged over the categories, this standard would be a category over the others; therefore there is no such standard.

It looks like a modus tollens argument: if there was a category of good, there would be a category over the others; there is no such category; therefore the good is not a category.

https://plato.stanford.edu/entries/categories/#AriRea goes over Aristotle's denial of a highest genus in terms of his doctrine of categories. It mentions unity and being in terms of this denial; I think the argument fits onto the good, here, as such (some think of the Form of the Good as "the One," for example).