On problem solving
Duncker, K. (1945). "On problem solving". Psychological Monographs, 58:5 (Whole No. 270).
To study productive thinking where it is most conspicuous in great achievements is certainly a temptation, and without a doubt, important information about the genesis of productive thought could be found in biographical material. A problem arises when a living creature has a goal but does not know how this goal is to be reached. Whenever one cannot go from the given situation to the desired situation simply by action, then there has to be recourse to thinking. The subjects (Ss), who were mostly students of universities or of colleges, were given various thinking problems, with the request that they think aloud. This instruction, "Think aloud", is not identical with the instruction to introspect which has been common in experiments on thought-processes. While the introspecter makes himself as thinking the object of his attention, the subject who is thinking aloud remains immediately directed to the problem, so to speak allowing his activity to become verbal. It is the shift of function of the components of a complex mathematical pattern—a shift which must so often occur if a certain structure is to be recognized in a given pattern—it is this restructuration, more precisely: this transformation of function within a system, which causes more or less difficulty for thinking, as one individual or another tries to find a mathematical proof. (PsycINFO Database Record (c) 2016 APA, all rights reserved)