Whereas the older psychology regarded reasoning as a force largely independent of associative habits, our present psychology defines reasoning as the organization and coöperation of habits rather than as a special activity. The present investigation furnishes evidence of the truth of the correlated theorem that "any disturbance whatsoever in the concrete particulars reasoned about will interfere somewhat with the reasoning, making it less correct or slower or both."A group of ninety-seven graduate students were given two sets of nine tasks in algebra. Each pair of tasks demanded the application of the same principle, but the concrete situation in one case was one made more or less familiar by associative habits, whereas in the other case the concrete particulars were somewhat altered. The per cent wrong or incomplete for the nine tasks in which customary associations were favored was 34.4, and for the nine in which some change was made, 54.2. From Psych Bulletin 19:09:00596. (PsycINFO Database Record (c) 2006 APA, all rights reserved)