All this is applicable to the intellectual faculties of man. There is a considerable difference between one person and another as regards these faculties, as is well known to philosophers. While one man can discover a certain thing by himself, another is never able to understand it, even if taught by means of all possible expressions and metaphors, and during a long period; his mind can in no way grasp it, his capacity is insufficient for it. This distinction is not unlimited. A boundary is undoubtedly set to the human mind which it cannot pass. There are things (beyond that boundary) which are acknowledged to be inaccessible to human understanding, and man does not show any desire to comprehend them, being aware that such knowledge is impossible, and that there are no means of overcoming the difficulty; e.g., we do not know the number of stars in heaven, whether the number is even or odd; we do not know the number of animals, minerals, or plants, and the like. There are other things, however, which man very much desires to know, and strenuous efforts to examine and to investigate them have been made by thinkers of all classes, and at all times. They differ and disagree, and constantly raise new doubts with regard to them, because their minds are bent on comprehending such things, that is to say, they are moved by desire; and every one of them believes that he has discovered the way leading to a true knowledge of the thing, although human reason is entirely unable to demonstrate the fact by convincing evidence.—For a proposition which can be proved by evidence is not subject to dispute, denial, or rejection; none but the ignorant would contradict it, and such contradiction is called “denial of a demonstrated proof.” Thus you find men who deny the spherical form of the earth, or the circular form of the line in which the stars move, and the like; such men are not considered in this treatise. This confusion prevails mostly in metaphysical subjects, less in problems relating to physics, and is entirely absent from the exact sciences. Alexander Aphrodisius said that there are three causes which prevent men from discovering the exact truth: first, arrogance and vainglory; secondly, the subtlety, depth, and difficulty of any subject which is being examined; thirdly, ignorance and want of capacity to comprehend what might be comprehended. These causes are enumerated by Alexander. At the present time there is a fourth cause not mentioned by him, because it did not then prevail, namely, habit and training. We naturally like what we have been accustomed to, and are attracted towards it. This may be observed amongst villagers; though they rarely enjoy the benefit of a douche or bath, and have few enjoyments, and pass a life of privation, they dislike town life and do not desire its pleasures, preferring the inferior things to which they are accustomed, to the better things to which they are strangers; it would give them no satisfaction to live in