33. Reasoning. Though the various forms of reasoning and the principles which they follow are the concern rather of psychology and logic than of a practical work on the writing of arguments, yet these sciences help us to understand the processes of the mind by which we convince first ourselves, and then other people, of the existence of facts, when for one reason or another direct testimony is wanting. Psychology describes the processes of reasoning as part of the activity of the mind, analyzes them into their parts, and shows their working. Logic is concerned rather with the forms of reasoning: its aim is to establish principles and rules the application of which will insure correct reasoning.
I shall first briefly and very simply sketch the underlying nature of the reasoning process as it is described by psychologists; then I shall pass on to a practical application of the principles thereby attained; next I shall set forth a few of the simplest and clearest of the processes of reasoning which have been worked out by logic; and, finally, I shall discuss each few of the best-recognized forms of false reasoning. From both the psychological description and the rules of logic we shall derive practical suggestions for establishing facts which may be needed in an argument.
The essential feature of the process of reasoning is that it proceeds from like to like, by breaking up whole facts and phenomena, and following out the implications or consequences of one or more of the parts.[24][!--Note--] For example, if I infer, when my dog comes out of a barnyard with an apologetic air, and with blood and feathers on his mouth, that he has been killing a hen, I am breaking up the whole phenomenon of the dog's appearance, and paying attention only to the blood and feathers on his head; and these lead me directly to similar appearances when I have caught him in the act. If I reason, Every student who can concentrate his attention can learn quickly, George Marston has a notable power of concentration, Therefore George Marston can learn quickly, I again break up the abstraction student, and the concrete fact George Marston, and pay attention in each to the single characteristic, concentration of attention. Thus by means of these similar parts of different wholes I pass from the assertion concerning the class as a whole to the assertion concerning the concrete case. This process first of analysis and then of abstraction of similars is the essential part of every act of reasoning.
In intuitive or unreasoned judgment, on the other hand, we jump to the conclusion without analyzing the intermediate steps. If I say, I have a feeling in my bones that it will rain to-morrow, or, it is borne in on me that our team will win, the sensations and ideas that I thus lump together are too subtle and too complex for analysis, and the conclusion, though it may prove sound, is not arrived at by reasoning. The difference between such intuitive and unreasoned judgments, and reasoning properly so called, lies in the absence or the presence of the intermediate step by which we consciously recognize and choose out some single attribute or characteristic of the fact or facts we are considering, and pass from that to other cases in which it occurs.
The skill of the reasoner therefore consists of two parts: first, the sagacity to pick out of the complex fact before him, the attribute or characteristic which is significant for his present purpose; and second, the large knowledge of the subject which will enable him to follow it into other cases in which it occurs with different circumstances, or, in other words, to follow a similarity through diverse cases. Darwin's great achievement in establishing the principle of evolution lay first in the scientific sagacity which flashed home on him, after years of patient study, that the one common fact in all the multitude of plants and animals is that in the struggle for existence by which all living beings persist, those who are best fitted to their circumstances survive; and second, in his rich knowledge of the world of nature, which made it possible for him to follow out this characteristic in all kinds of plants and animals, and so to reach the general law. But whether it be so world-sweeping a conclusion as his, or my conclusion that my dog has killed a hen, the process is the same: analysis or breaking up of the complex fact, and following out the consequences or implications of some selected part of it into other cases.
All reasoning thus reduces itself in the end to a process of passing from like to like: we notice that the present case is like other cases which we already know: then, since these cases have always in the past been accompanied by certain circumstances or consequences, we believe that the present case will also show these same circumstances or consequences. Whenever my dog has killed when the cases have been similar in the blood and feathers on his mouth; in this case he has blood and feathers on his mouth; therefore he must have killed a hen. Individual plants and animals survive which are fitted to their environment by special characteristics, and those which are not so fitted die; species of plants and animals, as well as individuals, show special adaptation to their environment; therefore species have survived through the same process of natural selection.
It follows that reasoning, whether it results in a general law or in concrete judgment, depends on the assumption that nature—and in nature we mean here the whole universe as we know it is uniform; that there are ties between facts which make it possible for us to be certain that if a given fact occurs, then another fact always occurs with it as an effect, or as a cause, or connected with it in some other manner. Without this certainty of the uniformity of things there would be no reasoning, and therefore no argument from indirect evidence. Huxley sets forth this fundamental truth clearly and impressively at the beginning of the first of his "Lectures on Evolution" (see p. 234).
For practical purposes the various types of this inference from similarity can be conveniently thrown into three groups. As will be obvious, there is no fixed and impassable line between them.
"If an inference relies upon a resemblance that is newly seen, rare, or doubtful, it is called an inference from analogy; if it is made upon the basis of an established classification, it is called a generalization; if it involves a variety of resemblances so combined as to bear upon a single point, it is usually or frequently called an inference from circumstantial evidence."[25][!--Note--]
I will take up each of these types and show how we use them in the practical work of argument. It will be seen that they vary greatly in certainty of results.
34. Reasoning from Analogy. Analogy in its most tenuous form is weak as a basis for an actual inference, though it is often effective as a means of expressing an intuitive judgment where the reasons are too subtle and diffused for formal explanation. When Lincoln in the middle of the Civil War said that men do not swap horses while they are crossing a stream, the analog, though subtle, was felt to be real. Popular adages and proverbs are common modes of expressing such deep-lying analogies: for example, "Where there is smoke there is fire"; "The slothful man saith, There is a lion in the way." Poetry too is full of these subtle, pregnant similarities which link things in some one aspect, but fail for all others.