"...two peculiarities which may be found by the curious in the works of Euclid."

A great deal is said in these days about the value or valuelessness of logic. In the main, indeed, logic is not a productive tool so much as a weapon of defence. A man building up an intellectual system has to build like Nehemiah, with the sword in one hand and the trowel in the other. The imagination, the constructive quality, is the trowel, and argument is the sword. A wide experience of actual intellectual affairs will lead most people to the conclusion that logic is mainly valuable as a weapon wherewith to exterminate logicians.

But though this may be true enough in practice, it scarcely clears up the position of logic in human affairs. Logic is a machine of the mind, and if it is used honestly it ought to bring out an honest conclusion. When people say that you can prove anything by logic, they are not using words in a fair sense. What they mean is that you can prove anything by bad logic. Deep in the mystic ingratitude of the soul of man there is an extraordinary tendency to use the name for an organ, when what is meant is the abuse or decay of that organ. Thus we speak of a man suffering from 'nerves,' which is about as sensible as talking about a man suffering from ten fingers. We speak of 'liver' and 'digestion' when we mean the failure of liver and the absence of digestion. And in the same manner we speak of the dangers of logic, when what we really mean is the danger of fallacy.

But the real point about the limitation of logic and the partial overthrow of logic by writers like Carlyle is deeper and somewhat different. The fault of the great mass of logicians is not that they bring out a false result, or, in other words, are not logicians at all. Their fault is that by an inevitable psychological habit they tend to forget that there are two parts of a logical process--the first the choosing of an assumption, and the second the arguing upon it; and humanity, if it devotes itself too persistently to the study of sound reasoning, has a certain tendency to lose the faculty of sound assumption. It is astonishing how constantly one may hear from rational and even rationalistic persons such a phrase as 'He did not prove the very thing with which he started,' or 'The whole of his case rested upon a pure assumption,' two peculiarities which may be found by the curious in the works of Euclid. It is astonishing, again, how constantly one hears rationalists arguing upon some deep topic, apparently without troubling about the deep assumptions involved, having lost their sense, as it were, of the real colour and character of a man's assumption. For instance, two men will argue about whether patriotism is a good thing and never discover until the end, if at all, that the cosmopolitan is basing his whole case upon the idea that man should, if he can, become as God, with equal sympathies and no prejudices, while the nationalist denies any such duty at the very start, and regards man as an animal who has preferences, as a bird has feathers.

-Twelve Types (1902)