“ Deficiency of understanding is called [pg 028] stupidity. It is just dulness in applying the law of causality , incapacity for the immediate apprehension of the concatenations of causes and effects, motives and actions. ”
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

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“But the limits of these conceptions must not be too sharply defined, for it is always that one function of the understanding by means of which all animals perceive objects in space, which, in its keenest form, appears now in the phenomena of nature, correctly inferring the unknown causes from the given effects, and providing the material from which the reason frames general rules as laws of nature; now inventing complicated and ingenious machines by adapting known causes to desired effects; now in the sphere of motives, seeing through and frustrating intrigues and machinations, or fitly disposing the motives and the men who are susceptible to them, setting them in motion, as machines are moved by levers and wheels, and directing them at will to the accomplishment of its ends. Deficiency of understanding is called [pg 028] stupidity. It is just dulness in applying the law of causality , incapacity for the immediate apprehension of the concatenations of causes and effects, motives and actions. A stupid person has no insight into the connection of natural phenomena, either when they follow their own course, or when they are intentionally combined, i.e. , are applied to machinery. Such a man readily believes in magic and miracles.” source