Do you know a related problem?  We can scarcely 
imagine a problem absolutely new, unlike and unrelated
to any formerly solved problem; but if such a problem
could exist, it would be insoluble. In fact, when solving
a problem, we always profit from previously solved problems, 
using their result, or their method, or the experience
we acquired solving them. And of course, the 
problems from which we profit must be in some way
related to our present problem. Hence the question: Do 
you know a related problem?
      there is usually no difficulty at all in recalling 
formerly solved problems which are more or less related to
our present one. On the contrary, we may find too many 
such problems and there may the difficulty in choosing a 
useful one. We have to look around round for closely related
problems; we LOOK AT THE UNKNOWN.
or we look for a formerly solved problem which is linked to our 
present one by GENERALISATION, SPECIALISATION, or ANALOGY.
     The question we discuss here aims at the mobilisation 
of our formerly acquired knowledge 
(PROGRESS AND ACHEIVEMENT,1). An 
essential part of our mathematical
knowledge is stored in the form of formerly proved
theorems. Hence the question: Do you know a theorem
that could be useful? This question may be particularity
suitable when our problem is a "problem to prove," that 
is, when we have to prove or disprove a proposed 
theorem.