Ethics is not an exact science

The theory of conduct (i.e., doing the right thing) is not an exact science. The reason, says Aristotle, is that every theory needs to be as exact as its subject matter. In the realm of conduct though, there are no exact rules. Consequently, a theory of conduct cannot have any exact rules either.

As an analogy, Aristotle points to matters of health. There are no exact rules for making a sick person healthy. Sometimes it's good to do an organ transplant, but sometimes it is not. It really depends on the circumstances. And since circumstances can be infinitely variable, there simply cannot be any solid, steady rules about making sick people healthy. A good doctor is not one who always assigned the textbook prescribed medication. On the contrary, a good doctor is one who knows when and where to do certain things, and when and where to not do such things.

The same goes for a theory of conduct. Sometimes it's good to do X, but sometimes it's good to do Y. It really depends on the circumstances. So when it comes to matters of conduct, we need to pay attention to the circumstances.

Still, Aristotle says we can do something to help us understand good conduct, and that's what he tries to do in the rest of the Nicomachean Ethics.