XXIII. In no art whatever, is it of any use, to have a physical knowledge of the implements, with, or by which, a man does his business. A man may be an excellent pilot, without being able to explain the directive virtue of the load-stone to the pole; and a great soldier, although he is totally ignorant of the physical component parts of gunpowder, and the metal with which military implements are made; and he may be a great painter, without knowing whether colours consist of intrinsic accidents, or whether they are produced by the various reflections of light. Nor does the being able to argue well upon these points, conduce in the least degree, to make a man either a pilot, a soldier, or a painter. But there is no necessity for my enlarging further, in order to extirpate this common error from the world, as the learned Doctor Martinez, has fully and effectually exploded it in his two volumes, intitled Medicina Sceptica. SECT. VI. XXIV. There is another common error respecting the subject-matter of this Essay, although the opinion built upon it, is rather better founded than the one we have just been speaking of, and that is, looking upon every man as wise, able, and skilful, who has studied a great deal. The labour of study, seldom makes any great improvements, if it is not bestowed on a clear and penetrating natural understanding; and may be compared to the toils and expences of cultivation, that are employed upon a poor and nearly barren soil, which seldom produce much fruit. Among mankind, are to be found the turtles and the eagles of the human species. The last, with a speedy and easy flight, ascend to the summit of Olympus; the first, after several days labour, shall not be able to reach to the top of a moderate hill. XXV. Much time employed in reading, furnishes a man with a deal of matter; but the being able to penetrate, or rightly comprehend that matter, is more a gift of nature, than a production from toil. There are some, who may be termed learned from their memories, but who are not so in virtue of their understandings, and who may be compared to marble tablets with inscriptions engraved on them, which display the letters, but do not perceive them. They are a kind of mental books, that are filled with many texts, which they no more comprehend, or have a right idea of the meaning of, than the leaves on which they are written. If you observe the use or application they make of the species they have acquired, you will perceive, they deduce no proper conclusions from what they have read, nor make any observations on the things they have been perusing, that are pertinent or applicable to the subject-matter of those things. Thus we see, that from the same species or set of ideas, may be formed good reasoning and bad; just as with the same sort of materials, you may build elegant palaces, and rustic habitations.