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mi; I'KINCII'I.K OK I,KST MOTION, BTC. IK7 The abstractly conceived principle of lessening KtYort is thus in no sense a forma tirr principle of mental development in the sense that a psychologist can deduce from it the way in which psychical processes are elaborated, even as the mathematician deduces from the physical principle of Least Action the actual paths that moving bodies must take. It may be a guiding thread or clue, 1 but a clue is not a forma- tive or synthetic principle. The thread of Ariadne cannot explain the killing of the Minotaur, it cannot explain the sword of Theseus. It cannot even explain how Theseus found the Minotaur ; it can only explain how he found a safe way back through the labyrinth. So it is with the abstract principle of Facilitation. It cannot explain either the actual discovery of psychological principles and laws, nor does it supply from its armoury any weapons for attacking them. It can only guide the psychologist over ground that he has already covered, and at best serve him as an analytical principle of rearrangement. Thus we might conceivably systematise the subject-matter of Psychology by answering in detail the following question : ' What are the conditions that facilitate the exercise of the various psychical activities, retention, reproduction, discrimination, association, etc.?' The illusive explanatory power of an abstract product of this sort is due to its undeniable generality, to the fact that the common element it expresses is a general characteristic (if the whole process iu all its parts. Be it elimination or elaboration, lessening of effort does take place. The inference is then made that it must therefore be an essential factor in mental process. This may possibly happen to be the case, thus it might have happened to be true that the direct impulse or aim of mental process was at all costs to lessen and synthesis, but from a process of mere comparison ; (2) in its function. The true abstract universal, as could be inferred in advance from its mode of genesis, is explanatory, and is the genuine universal of all abstract Science after it has reached the explanatory stage. The mere abstract universal is at best descriptive and is the universal propt-r to a system of classification. In illustration of these distinctions we might take the two processes of elimination and elaboration as they take place in mental development. A mere abstract universal stating an element these two processes share in common we have found in the abstract conception of lessening effort. A true abstract universal giving unity to the two processes as they actually take place might be found in Control, when by control I mean the reinforcing or inhibiting of motor tendencies, ideal or corporeal, in view of satisfying some desire or carry- ing out some design. In elimination, Control exercises its inhibitory function, in Elaboration, its reinforcing function. 1 Cf. Helmholtz, Ul>,',- </!>' jJitisihilixi-li' ll> <{ ntung ties Print //< </< r kleinsten Wirkuny, pp. 142, 143.