Thomson says: “The system of anatomy which has immortalized the name of Oken, is the consequence of a flash of anticipation which glanced through his mind when he picked up in a chance walk the skull of a deer, bleached and disintegrated by the weather, and exclaimed, after a glance, ‘It is part of a vertebral column.’ When Newton saw the apple fall, the anticipatory question flashed through his mind, ‘Why do not the heavenly bodies fall like this apple?’ In neither case had accident any important share; Newton and Oken were prepared by the deepest previous study to seize upon the unimportant fact offered to them, and show how important it might become; and if the apple and the deer-skull had been wanting, some other falling body, or some other skull, would have touched the string so ready to vibrate. But in each case there was a great step of anticipation; Oken thought he saw the type of the whole skeleton in a single vertebra, whilst Newton conceived at once that the whole universe was full of bodies tending to fall.”
Passing from the consideration of Inductive Reasoning to that of Deductive Reasoning we find ourselves confronted with an entirely opposite condition. As Brooks says: “The two methods of reasoning are the reverse of each other. One goes from particulars to generals; the other from generals to particulars. One is a process of analysis; the other is a process of synthesis. One rises from facts to laws; the other descends from laws to facts. Each is independent of the other; and each is a valid and essential method of inference.”
_Deductive Reasoning_ is, as we have seen, dependent upon the process of deriving a particular truth from a general law, principle or truth, upon the fundamental axiom that: “What is true of the whole is true of its parts.” It is an analytical process, just as Inductive Reasoning is synthetical. It is a descending process, just as Inductive Reasoning is ascending.
Halleck says of Deductive Reasoning: “After induction has classified certain phenomena and thus given us a major premise, we proceed deductively to apply the inference to any new specimen that can be shown to belong to that class. Induction hands over to deduction a ready-made major premise, _e.g._ ‘_All scorpions are dangerous_.’ Deduction takes this as a fact, making no inquiry about its truth. When a new object is presented, say a possible scorpion, the only troublesome step is to decide whether the object is really a scorpion. This may be a severe task on judgment. The average inhabitant of the temperate zone would probably not care to risk a hundred dollars on his ability to distinguish a scorpion from a centipede, or from twenty or thirty other creatures bearing some resemblance to a scorpion. Here there must be accurately formed concepts and sound judgment must be used in comparing them. As soon as we decide that the object is really a scorpion, we complete the deduction in this way:–‘_All scorpions are dangerous_; _this creature is a scorpion_; _this creature is dangerous_.’ The reasoning of early life must be necessarily inductive. The mind is then forming general conclusions from the examination of individual phenomena. Only after general laws have been laid down, after objects have been classified, after major premises have been formed, can deduction be employed.”
What is called _Reasoning by Analogy_ is really but a higher degree of Generalization. It is based upon the idea that if two or more things resemble each other in many particulars, they are apt to resemble each other in other particulars. Some have expressed the principle as follows: “Things that have some things in common have other things in common.” Or as Jevons states it: “The rule for reasoning by analogy is that if two or more things resemble each other in many points, they will probably resemble each other also in more points.”
This form of reasoning, while quite common and quite convenient, is also very dangerous. It affords many opportunities for making false inferences. As Jevons says: “In many cases Reasoning by Analogy is found to be a very uncertain guide. In some cases unfortunate mistakes are committed. Children are sometimes killed by gathering and eating poisonous berries, wrongly inferring that they can be eaten, because other berries, of a somewhat similar appearance, have been found agreeable and harmless. Poisonous toadstools are occasionally mistaken for mushrooms, especially by people not accustomed to gather them…. There is no way in which we can really assure ourselves that we are arguing safely by analogy. The only rule that can be given is this, _that the more things resemble each other, the more likely is it that they are the same in other respects, especially in points closely connected with those observed_.”