Which one should I use?

In the following case, which should I use?

(I want to state it in general.)

·It’s important to separate things you understand and things you don’t understand.

·It’s important to separate the things you understand and the things you don’t understand.

I know that we use “no article” when the group doesn’t have a clear boundary, but we can also use “the” to group something especially when we compare it with other groups.

In the case I showed, I think there isn’t a clear line between these two ways for articles. So, I don’t know which I should use in this case.