Whence the notion of unending science?

In nearly every popular account of science and the scientific method, we
are
told that "we will never understand everything". My question is simply,
why
do we say that? Complete knowledge does not seem to violate any logical
principle; etc.

I have put this question to scientists many times; the only actual answer
I
have ever received is that we know that past science was always wrong in
various respects, so any present science is also. So is it just an
inductive
argument?

And when did we start saying this?

Any insight appreciated.

Ken