Ill-defined definition
Ill-defined means there are no formal rules, or procedures allowing us to assert with a proof whether or not we have reached a desired solution for a given problem. It happens when the latter is unparalleled, urgent and pressing but rather "felt" than properly "understood" and articulated. In such cases we are compelled to resort mostly to heuristic approach because it is futile to ask for any known "algorithms" or precisely defined "scientific" methods to attack a problem in purely rationalistic manner. The majority of educational problems mentioned in a Draft Memory Note (above all — "training Teachers of Teachers") obviously falls into this category.