The quality of observation

or: "The necessary knowledge is that of what to observe."

"The analytical power should not be confounded with simple ingenuity; for while the analyst is necessarily ingenious, the ingenious man often remarkably incapable of analysis. The constructive or combining power […] has been so frequently seen in those whose intellect bordered otherwise upon idiocy, as to have attracted general observation among writers on morals.
Between ingenuity and the analytic ability there exists a difference far greater, indeed, than that between the fancy and the imagination, but of a character very strictly analogous. It will found, in fact, that the ingenious are always fanciful, and the truly imaginative never otherwise than analytic."

(E.A.Poe, The murders in the Rue Morgue)