REASONING, or INFERENCE, is the mental process in which we advance from some known fact or principle to the truth of some other fact which is different from the start ing-point. The basis for the transition is always found in the knowledge from which we set out. This is taken or assumed to be real, and in it is found the ground and justification for the advance to something else. The differentia of reasoning thus appears to be mediation; when we reason we infer that something is true because something else is true. Knowledge derived from reason ing may, therefore, be termed mediate, as opposed to the immediate knowledge obtained from sense perception and memory. The ques tion at once arises how any mental content can justify an advance to something different from itself ? How are we ever warranted in passing from the known to the unknown? This is not merely the question that Mill raised as to whether all syllogistic reasoning — all advance from premises to conclusion — was not a imago principii; but it concerns all reasoning, induc tive and deductive alike. The dilemma is that if the result is not contained in the starting point the advance does not seem to be justi fied; if it is already present, the reasoning shows nothing new. The view, of Leibnitz was that all reasoning is analysis, a drawing out and fuller explication of the original datum of the mind. Kant pointed out that thinking involves also synthesis, new constructions and additions to the material from which it starts, and he takes as the fundamental problem of his 'Critique of Pure Reason' the question how such synthetic judgments are possible. His an swer is essentially identical with that which Aristotle gave, namely, that the mind or reason itself enters into the process as a premise — or, in other words, that it is through the creative activity of the mind that the new truth is reached. Whether or not one can connect one fact with another in a logical way depends upon one's intellectual ability to dis cover points of essential resemblance or iden tity between facts. The good reasoner is he who can look beneath the surface and detect identities that are not at once obvious, as New ton, for example, did when he reasoned from the fall of the apple to the movements of the heavenly bodies. Reasoning, then, may be de fined as the process of discovering essential semblances or points of identity between things.
It follows from what has been said that reasoning is not a process or function of mind that can go on apart from experience. The
thinkers of the modern period down almost to the end of the 18th century continued to be lieve that reason was a kind of special organ or faculty that could yield truth of the highest order of certainty quite apart from ordinary experience. It was Kant who first clearly and incontestably showed the impossibility of de riving knowledge from reason taken in ab straction from ordinary sense experience. Kant, however, uses the term (Verstand) for the thinking faculties as em ployed in interpreting experience, and reserves the name (Vernunft) for the vain and illusory attempt of thought to operate in inde pendence of any given material of experience. Apart from this terminology, however, which has not been generally followed, the result of Kant's teaching was to exhibit the close and essential connection that exists between think ing and sense-perception: on the one hand, thought is empty apart from the material of sense-perception, and, on the other, what we call ordinary perceptive experience is constantly in terwoven with more or less explicit processes of reasoning. Reasoning does not go on in a vacuum, nor is it a separate and distinct func tion of mind that in some mysterious way spins truth out of itself. But reason is on one side a universal function of receptivity; it receives its material from every channel of experience, and is itself just the unifying, co-ordinating and systematizing life of experience. De duction and induction are often spoken of as if they were distinct species of reasoning. Reasoning, however, is always one and the same process. It consists, as we have seen, in con necting parts of experience by the discovery of some identical element in them. This identity, as present in various particulars, we may speak of as a universal, or general, principle, and, therefore, say that when we reason we unite particulars through a general law or principle. Ilow, the difference between deduction and in duction is a difference in the starting-point and in the direction in which we proceed. If we are already in possession of the general law, and set out to apply it to particular cases, we are using deduction. If, however, our starting-point is the particular instances, then we reason in ductively to discover the universal law of con nection. In both cases the structure of the completed inference is the same, and consists in the connection of particulars, in virtue of our insight into the universal law or principle ex pressed in them.