When a scientist says something is impossible?

A quote by sir Arthur C. Clarke could perhaps answer your question.

"When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."

The only way of discovering the limits of the possible is to venture a little way past them into the impossible.

However, in terms of logic, it would only mean that there are really some things that Science cannot explain.