The World as Will and Idea, Vol. I — Arthur Schopenhauer, trans. Haldane & Kemp

Deficiency of understanding we call stupidity: deficiency in the application of reason to practice we shall recognise later as foolishness : deficiency of judgment as sillines8,_ and lastly, partial oF entire deficiency of as madness. But each of these will he considered in its own place. That which is correctly known, by, is truih^^ that is, an abstract judgment on sufficient ^unds (Essay on the I’rinciple of Sufficient lieason, §29 and following paragraphs) ; that which is correctly known by understanding is reality, that is correct inference from effect oh the immediate object to its cause. Error is opposed to truth, as deception of the reason: illusion is opposed to reality, as deception of the underda/ndiug. The full discussion of all this will be found in the first chapter of my essay on Light and Cplour. Illusion takes place when the same effect may be attributed to two causes, of which one occurs very frequently, the other very seldom ; the understanding having no data to decide which of these two causes operates in any particular case, — for their effects are exactly alike, — always assumes the presence of the commoner cause, and as the activity of the understanding is not reflective and discursive, but direct and immediate, this false cause appears before us as a perceived object, whereas it is merely illusion. I have explained in the essay referred to, how in this way double sight and double feeling take place if the organs of sense are brought into an unusual position ; and have thus given an incontrovertible proof that perception exists only through and for the understanding. As additional examples of such illusions or deceptions of the understanding, we may mention the broken appearTHE WORLD AS IDEA.

3J 3of.a stick dipped in water; the reflections in s^ericaj, mirror^ which, when the surface is convex appear somewliatluehind it, and when the surface is concave appear aTo^ waj in front of it. To this clas^ also hdongs the apparently greater extension of the raoonr,at the^hpriMii than at the zenith.

This appearance is not optical, for as tEe micrometre proves, the eye receives the image of the moon at the zenith, at an even greater angle of vision than at the horizon.

The mistake is due to the understanding, which assumes that the cause of the feebler light of the moon and of all stars at the horizon is that they are further off, thus treating them as earthly objects, according to the laws of atmospheric perspective, and therefore it takes the moon to be much larger at the horizon than at the zenith, and also regards the vault of heaven as more extended or flattened out at the horizon.

The same false application of the laws of atmospheric perspective leads us to suppose that very high mountains, whose summits alone are visible in pure transparent air, are much nearer than they really are, and therefore not so high as they are; for example, Mont Blanc seen frpm^ Salenche. All such illusions are immediately present to us as perceptions, and cannot be dispelled by any arguments of the reason.

Reason can only prevent error, that is, a judgment on insufficient grounds, by opposing to it a truth ; as for example, the abstract knowledge that the cause of the weaker light of the moon and the stars at the horizon is not greater distance, but the denser atmosphere ; but in all the cases we have referred to, the illusion remains in spite of every abstract explanation.

For the understanding is in itself, even in the case of man, irrational, and is completely and sharply distinguished from the reason, which is a faculty, of knowledge that belongs to man alone. The reaso'n can only know; perception remains free from its influence and belongs to the understanding alone.