A dialectician (logician in the narrower sense) is neither a grammarian nor an encyclopedia of the best information on every subject. His office consists in deciding whether certain theorems are arguments or not. An Argument is an act or product of rational imagination. Theorems which purport to be arguments, but are not, are Fallacies.
A fallacy is not merely a bad argument—it is no argument at all. Quite apart from fallacy there is a goodness and badness in arguments, but with this discrimination the dialectician (as such) has nothing to do. Only persons experienced in the matter are competent to decide between good and bad arguments. Hence when the quality of an argument is in question the dialectician takes no part in the debate: he is neither combatant nor umpire. He is at most an impartial president whose chief duty is to see that people do not debate about mere words and foregone conclusions. Granting that a theorem has the qualities of an argument, the dialectician is not competent to say that it is improper or too trivial to be discussed. He is not a judge of what people ought to be interested in.
From his better knowledge of what constitutes rational prediction, a dialectician may offer his services to disentangle and render explicit involved and partial arguments. Many people reason well who are yet unable to express themselves coherently. A dialectician should be able to reconstruct an argument from the slightest hint, as a naturalist imagines an animal from a single bone. In ordinary reasoning the arguments are seldom fully expressed, and the reasoners themselves are not always quite conscious of the premises from which they argue. All such suppressed and overlooked assumptions should be brought to light by dialectic, the aim of which is to render reason as self-conscious as possible.
Though a dialectician need not be an expert in any department of knowledge, he must know the facts on which an argument is built, otherwise he may be deceived by equivocal language. Reverting to the instance of the vase—the dialectician must have seen both the whole vase and the broken vase, but he need not have any opinion as to whether the proposed handle is the most suitable, or not. That must be left to those who are familiar with vases and who are interested in the restoration of the one in question.
The definition here given of the scope and office of dialectic may appear to some too modest. But in reality there is a great deal involved in it. Philosophers have been discussing Reason for twenty centuries or more, and have not produced a satisfactory definition of it. Consequently they cannot decide with any confidence whether a theorem is an argument or a fallacy. The cleverest of them give their sanction to theorems that are demonstrably fallacies. They are evidently judging more by ear than by rule. All this causes confusion of mind and waste of energy.
Dialectic takes its general idea of reason from the higher analysis of logic, and brings the general idea to bear on concrete arguments. A dialectician makes a collection of theorems for study just as a botanist makes a collection of plants. He sorts them out into convenient classes, separates the valid or useful from the erroneous and misleading, studies the relation of language to argument and the influence for good or ill that words have upon rational thought.
From the example of the vase cited above it will be seen that in every act of reason two principal things are requisite. There must be something wholly known (or comparatively well known) and something less well known, and the reasoning or argument consists in ideally completing the latter on the model of the former. If we would predict the coming of a future season of the year we must have a picture in the mind of all the seasons in the order in which they occur. If we would go straight to a place on the surface of the earth we must have a plan of the way in our imagination. If we would predict the effect of a drug on an animal body we must have previously noticed the effect it has produced—and so on. Neither the mind nor intellect supplies spontaneously any of these models; they are all formed out of actual experience remembered and recollected. When they have been refined into extremely general ideas they are apt to be taken for innate tendencies of the intellect, as Kant erroneously thought. They are not so; all we know of the intellect is consistent with the belief that it begins with pure plasm without a trace of idea, and is absolutely indifferent to the imprints it may receive. Doctrines of innate ideas—innate forms of thought or categories—innate 'principles' of various kinds—are devices of metaphysicians to cover the weakness of their theories.
The two main parts of an argument divide naturally into four subdivisions. There is the thing argued about (corresponding to the broken vase); there is the ideal extension or restoration; in the model we reason from there are the parts corresponding to each of these. I propose to take terms for these four parts from one of the most important, formal and correct modes of reasoning—the application of a precedent or statute to a case in Law.