Opinion involves belief (for without belief in what we opine we cannot have an opinion), and in the brutes though we often find imagination we never find belief.
If, then, there is some end of the things we do, which we desire for its own sake (everything else being desired for the sake of this), and if we do not choose everything for the sake of something else (for at that rate the process would go on to infinity, so that our desire would be empty and vain), clearly this must be the good and the chief good.
The so-called Pythagoreans, who were the first to take up mathematics, not only advanced this subject, but saturated with it, they fancied that the principles of mathematics were the principles of all things.
Error is multiform (for evil is a form of the unlimited, as in the old Pythagorean imagery, and good of the limited), whereas success is possible in one way only (which is why it is easy to fail and difficult to succeed - easy to miss the target and difficult to hit it); so this is another reason why excess and deficiency are a mark of vice, and observance of the mean a mark of virtue: Goodness is simple, badness is manifold.
Xenophanes states that the fire in Lipara once failed for sixteen years, but returned in the seventeenth year. They say that the lava-stream in Etna is neither flaming nor continuous, but returns only after an interval of many years.
We maintain, and have said in the Ethics, if the arguments there adduced are of any value, that happiness is the realization and perfect exercise of virtue, and this not conditional, but absolute. And I used the term 'conditional' to express that which is indispensable, and 'absolute' to express that which is good in itself.