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] 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.