17. uncritical mind may be led into error by either induction or deduction, but most of our mistakes, prejudices and wrong ideas are due to our careless use of the inductive method. People observe a few facts and then "jump" to a conclusion. Their minds being untrained, they often cling to their conclusions with great obstinacy and refuse to listen when anybody seeks to enlighten them. Hence we find people still carrying horse-chestnuts in their pockets to prevent rheumatism, wearing amber beads about their necks to ward off a sore throat, confident that a "wet" moon indicates rain, or sure that olive oil, being "oil," cannot possibly be palatable as food.
Since all inductions are liable to imperfections, the scientific man submits them to tests before he accepts them as truths. In certain fields, especially chemis try, physics and bacteriology, inductions are tested in the laboratory. In other sciences, such as economics and sociology, laboratory tests are impossible. Stu dents of these sciences can test their conclusions only by repeated observations of events in the actual world, but here satisfactory tests are difficult because the same event.never occurs twice under exactly the same conditions.
The scientific man, seeking an explanation of a phe nomenon frequently recurring in nature, constructs what is called an hypothesis, which is merely a guess at the truth. He assumes that the phenomenon is the result of certain conditions, and that when these con ditions exist this phenomenon will inevitably follow ; then he proceeds to make observations to confirm his hypothesis if possible. If he finds that his guess is correct so far as all his observations go and that the phenomenon never occurs excepting under the condi tions which he has "guessed" to be necessary, he will conclude that he has reached the truth. Then his hypothesis will have risen to the dignity of a theory or law of nature.
18. theory of evolution by natural selection was at first only a scientific guess on Dar win's part. It occurred to him as a reasonable hy pothesis when he read a book by Thomas Malthus on the "Theory of Population," in which Malthus showed that the tendency of population was to increase faster than the food supply, so that the weakest perished and the strongest survived. Darwin made observations of animal life with infinite patience, and it is generally believed that the truth of his hypothesis has been proved. It is now called the "theory of evolution."
For many years men observed such phenomena as the falling apple, but nobody so far as we know, sought for an explanation until in the seventeenth cen tury Newton's -curiosity was aroused. This is really not strange, for the human mind is least curious about the phenomena with which it is most familiar. Air and water, the two substances most essential to our life and health, received no scientific attention un til within recent years; in fact, scientists still seem to be in doubt, not as to the nature of air, but as to the evil effects of impure air and the best method for its purification by ventilation.
The theory of gravitation was first an hypothesis or cruess in Newton's mind. "If I can assume that bod ies of matter," he said to himself, "attract each other mutually in proportion to their size and density and inversely as the squares of their distance, the laws of motion being true, then I should be able to calculate aright the orbit of each plank in the solar system." _Newton first tested the truth of his law by a study of the moon's orbit, but at that time, 1665, the exact distance of the earth from the moon was not blown and his calculations were not entirely satisfactory. Before his death, however, the size of the earth and its distance from the moon had been accurately &ter mined and Newton had the satisfaction of Imowing that his law of gravitation had been verified. In 1845, a young French astronomer named Leverrier, puz zling over the erratic behavior of the planet Uranus, came to the conclusion that it must be under the in fluence of an undiscovered planet located in a certain part of the heavens. He bad no telescope powerful enough to bring this unlil-lown planet into the field of vision, but Ile wrote to an astronomer in Berlin and told him in what direction to turn his powerful tele scope. The German astronomer followed directions and within half an hour found the new planet almost exactly where Leverrier bad indicated. It was the planet we now know as Neptune. It was discovered as the result of deductive or a priori reasoning based on an induction, namely, Newton's law of gravitation.