THEORY, a word expressing the scientific process of generalization under various aspects.
Theory is, in the first place, opposed to fact, or matter of fact, and signifies that a certain class of facts have been generalized and brought into a single comprehensive statement. It thus corresponds to a principle, general truth, or law of nature. That a half-inflated bladder hung before the fire is expanded till it bursts is a matter of fact; that bodies generally are expanded by heat is the theory or general principle, compre hending the whole class of facts. To give the theory of a fact in this sense of the word is to give its general law; this is also called its explanation, and sometimes its cause. See CAUSE.
Theory, in the next place, is opposed to hypothesis (q.v.). A fact may for a time be referred to a hypothetical or assumed principle; endeavors being meanwhile made to remove the hypothetical character by proving or disproving the principle. The vortices of Descartes was a hypothesis to account for planetary motions; while Newton's view, that gravity might be the cause of these motions, was in the first instance a hypothesis. The Cartesian doctrine was disproved and abandoned; the Newtonian was fully verified, and, ceasing to be a hypothesis, became a theory.
Lastly, theory is opposed to practice. The theory of a subject is the knowledge or explanation of it; the practice is making some use of it. Physiology is theory; physic, or medicine, is practice. In practical matters there are two modes of procedure which are still further illustrative of the distinction now in hand. The knowledge possessed
by a worker in any art may be empirical, experimental, rule-of-thumb—that is, it may be gathered by actual experience in the particular operation. The seaman's knowledge of the prognostics of weather, and the cook's art in boiling and roasting, are usually of this kind. On the other hand the worker's knowledge may be obtained from theory, in other words, from general principles or laws scientifically ascertained; as when the theory of the winds and the law of storms are employed to predict the weather; when the cook roasts and boils according to the known temperature for coagulating albumen; and when a physician prescribes a dietary grounded on a chemical analysis of the food and of the tissues to be maintained. Great caution is required in the employment of such theoretical knowledge in the arts and in practical affairs. It is not enough that the theories are fully established; we must also know all the conditions of the case, so as to allow for every agent operating to produce or to mar the effect. That a cannon-ball should describe a parabola, is a correct theoretical inference from gravity and the laws of motion; but the resistance of the air, a distinct agency, makes it untrue in fact, and therefore misguiding in practice. When this resistance is allowed for, the theory is complete, and its application will no longer disappoint the operator. See DEDUCTION.