Full Idea
What is contrary to a good thing is necessarily bad, as we see with health and sickness. But the contrary of bad is sometimes good, sometimes not, as we see with excess, opposed by both deficiency and moderation.
Gist of Idea
The contrary of good is bad, but the contrary of bad is either good or another evil
Source
Aristotle (Categories [c.331 BCE], 13b36)
Book Reference
Aristotle: 'Categories and De Interpretatione', ed/tr. Ackrill,J.R. [OUP 1963], p.38