[mks]

For me this means you have to be able to reason about your code. When you write a piece of code you need to understand what it does, how it behaves and why it behaves the way it does.

It is not uncommon to reveal bugs during code review just by looking at code when the author claims they have tested everything and it works fine. Of course they have missed some test case, but how do you make sure your quality assurance does not miss something as well?

I believe it is cheaper and faster to understand the code, side effects and interactions than to rely only on testing (which is immensely important of course).

In order to grow as a developer one has to understand there is no magic - you need to strive for understanding.