It is possible to satisfy the condition?   Is the condition
sufficient to determine the unknown?  Or is it insufficient? 
Or redundant?  Or contradictory?
     These questions are often useful at an early stage when
they do not need a final answer but just provisional
answer, a guess.  For example, see sections 8, 18.
     It is good to foresee any feature of the result for which 
we work.  When we have some idea of what we can 
-expect, we know better in which direction we should go.
Now, an important feature of a problem is the number
of solution of which it admits. Most interesting among
problems are those which admit of just one solution; we
are inclined to consider problems with a uniquely determined
solution as the only "reasonable" problems. Is our
problem, in this sense, "reasonable"? If we can answer
this question, even by a plausible guess, our interest in
the problem increases and we can work better.
     Is our problem "reasonable?" This question is useful
at an early stage of our work if we can answer it easily.
If the answer is difficult to obtain, the trouble we have 
in obtaining it may outweigh the gain in interest. The
same is true of the question "is it possible to satisfy the 
condition?" and the allied question of our list. We
should put them because the answer might be easy and
plausible, but we should not insist on them when the
answer seems to be difficult or obscure.
     The corresponding questions for "problems to prove" 
are:  is it likely that the proposition is true? or is it more
likely that is false? The way the question is put shows
clearly that only a guess, a plausible provisional answer,
is expected.