Peruse the notes on heuristics. They provide the background on this "mental mathematical block" that has been placed squarely upon the noggin of modern computer science's pride-swollen head.
This modern misinterpretation and misunderstanding of the "heuristic" problem-statement has left the field at a "great loss".
Clever-insights, empirical-study, and innovative-intution make problems such as that of "finding a single cliqué" or "enumerating all of the cliqués"
in a simple undirected-graph amenable.
In short, these are not "impossible". And truly, by the standards of accomplished mathematicians, these are not "difficult". Which is a synonym for "intractable".