It's describing how a humble scientist would not say he knows everything about something unless he has studied every aspect of it and left absolutely nothing nothing out. So he really knew everything about that thing. Since that is impossible, a scientist will never confine himself to saying something like he knows something. But as far as he knows it's this and that.
Of course, that's not to be confused with an expert, who knows everything about nothing.

And that's my friends, is what we call "balagha" in Arabic. I can write couple of pages on the few words mentioned in this post.