§ 51. Each Science has for its Guiding Thread one of the Forms of the Principle of Sufficient Reason in preference to the others.
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Schopenhauer, Arthur and Hillebrand, Karl, Mme.
As the question why always demands a sufficient reason, and as it is the connection of its notions according to the principle of sufficient reason which distinguishes science from a mere aggregate of notions, we have called that why the parent of all science (§ 4). In each science, moreover, we find one of the forms of that principle predominating over the others as its guiding-thread. Thus in pure Mathematics the reason of being is the chief guiding-thread (although the exposition of the proofs proceeds according to the reason of knowing only); in applied Mathematics the law of causality appears together with it, but in Physics, Chemistry, Geology, &c., that law entirely predominates. The principle of sufficient reason in knowing finds vigorous application throughout all the sciences, for in all of them the particular is known through the general; but in Botany, Zoology, Mineralogy, and other classifying sciences, it is the chief guide and predominates absolutely. The law of motives (motivation) is the chief guide in History, Politics, Pragmatic Psychology, &c. &c., when we consider all motives and maxims, whatever they may be, as data for explaining actions—but when we make those motives and maxims the object-matter of investigation from the point of view of their value and origin, the law of motives becomes the guide to Ethics. In my chief work will be found the highest classification of the sciences according to this principle.[160]