LOGIC is the systematic study of the general conditions of valid inference. There are other conceptions of the scope of logic. The principal rival conceptions are opposite ire character. One view would limit logic to the study of what is sometimes called formal logic and sometimes deductive logic. This view finds favour among mathematically-minded writers who wish to bring logic and mathematics into intimate connection. But while there is room for a border science between the two (see LOGISTIC) there is no good reason for excluding from the scope of logic any of the main types of reasoning. On the contrary, there are obvious advantages in including in one systematic study a comprehensive survey of all the main types of inference. The only other view that need be considered goes to the opposite extreme and would include under logic the psychology of thinking and the theory of knowledge. The thorough study of logic cer tainly requires attention to both psychological and epistemological problems. But that is a different thing from fusing them all to gether. This is only too likely to end in confusion, and to lead to the neglect of the properly logical problems. The tendency now under consideration is natural enough when regard is had to the historical unity of philosophy, and its resistance to differenti ation into separate studies. There are still philosophers of the old school who look upon this differentiation with as much dis approval as some British statesmen of the old school feel about the increasing autonomy of various parts of the British empire. But since both psychology and theory of knowledge are now recognized as well established, autonomous disciplines, it would be sheer waste of space and time to discuss their special prob lems all over again under the heading of logic. The scope of logic, then, as here conceived, is that indicated in the opening sentence of this article. But in order to understand that definition or description of logic, it is necessary to understand the terms employed in the opening statement. Let us begin with the term "inference," which retains in logic much of its etymological mean ing.
Inference, Judgment, Proposition, Belief, Knowledge.— Inference is the act or process of deriving one judgment or proposition from another or from others. The judgment or propo sition so derived is also called an inference; and for the most part we shall use the term inference in this more concrete sense, that is, as meaning a derived judgment or proposition. By a "proposition" is here meant a judgment expressed in words, the judgment itself being the actual thought or belief in the mind, which may or may not be expressed in a proposition, though we cannot discuss it until it is so expressed. But this needs further elucidation. All beliefs and all knowledge consist of judgments. For our present purpose the term belief is wider than, and in cludes the term knowledge. All knowledge is belief ; but not all belief is knowledge. In ordinary usage it would probably be denied that knowledge is belief. But that is not quite correct.
In ordinary usage the term "belief" is restricted to what we do not know but still accept as true. This identification of belief with what may more correctly be called mere belief is awkward. For, strictly speaking, if we can believe even what we do not know for certain, we most surely believe what we do know. So that all knowledge is belief, though there is also a mere belief of what cannot claim to be knowledge, that is, adequately justified belief. In the sense explained, then, judgment may be identified with belief, with all that we know for certain, or think as more or less probable, or merely believe without any evidence worth speaking about. But what exactly is judgment? Judgment is essentially a process of intellectual orientation. Life is only possible by continuous adjustment to surroundings, and therefore requires that living beings should take their bear ings so as to adapt their actions to their environment. At the lower levels of life the orientation is rather blind and merely in structive, though never merely mechanical. The method pursued is that of "trial and error," which frequently leads to improved adaptation. In this way even the lowliest organisms learn from experience. At the human level, however, a new form of orienta tion emerges. It is intellectual; it consists of judgments. Intel lectual orientation is, to begin with, a new instrument in the struggle for existence. It is an enormous improvement on its predecessor in the extent to which it enables man to learn from past experience. not only from his own experience but also from that of others. For it is more articulate, more clear-sighted. Situations already experienced, and successful methods of dealing with them when once discovered, are retained in the form of ideas which are helpful in new situations of the same type. The connections between things and events are not merely felt vaguely, as at the lower levels of life, but are apprehended more or less closely, and are frequently articulated in general ideas or laws of interconnection. These can then be applied to imaginary as well as to real cases. So there arise purely imaginary experiments, as distinguished from actual experiments, by the method of trial and error. The biological advantage of such imaginary experi ments is obvious in all cases in which actual experiments are beset with danger. In course of time intellectual interests extend beyond the pressing needs of the moment. The order and con nections of natural phenomena are studied in a spirit of dis interestedness, and thereby render possible an orientation that is characterized by greater depth of insight, and an enormously in creased range of foresight. And all this intellectual work consists of judgments interconnected by "why" and "wherefore," "be cause" and "therefore," which constitute inference or reasoning. It is the main business of logic to trace the principal types of inference and the general conditions of their validity.