Sufficient Reason, Principle of

(principium sive lex rationis sufficientis), the principle in logic according to which every judgment, except judgments of immediate sense perception, axioms, and definitions, must be substantiated (proved) in the sense that sufficient reason to accept the truth of this judgment or proposition must be adduced.

Acceptable as sufficient reasons may be the following: axioms (postulates or principles), definitions, verified or authenticated judgments of direct sense perception, and deductive judgments that have already been substantiated by proofs. Notice must be taken of the fact that in practice scientific and everyday thinking may not explicitly formulate certain propositions providing the sufficient reasons—for example, axioms, scientific laws, and generally accepted notions. Likewise, every step of an argument, leading from the basic propositions to the proposition to be substantiated, is not always shown. This does not violate the principle of sufficient reason so long as the form of the argument and the propositions supporting it (including those that have been left out but that can be shown if necessary) do in fact substantiate the conclusion.

The principle of sufficient reason in fact lay at the basis of all theories of logic in ancient times, in the Middle Ages, and in modern times, although it was not explicitly formulated as a special principle until the time of G. W. von Leibniz, who vested it not only with a logical meaning (relating to thought) but also with an ontological one (relating to existence): “no phenomenon can be true or real, and no assertion justified, without showing the sufficient reason why things are just the way they are and no other way” (Izbr. filos. soch., Moscow, 1908, p. 347). In the subsequent development of the history of logic, the principle of sufficient reason came to be under-stood as a purely logical principle. With the development of mathematical logic it became clear that its nature is entirely dependent on content—it cannot be represented as a formula in any logical calculus.

The principle of sufficient reason is usually realized in any particular line of reasoning with some degree of approximation. Some profound questions arise in this connection, particularly regarding the means for arriving at reliable conclusions in sciences based on observation and experiment or on extensive use of probable and inductive arguments. The problem of providing a reason for any judgment or proposition belongs in all of its fullness to the realm of the theory of knowledge, which answers questions about the connection between providing propositions with logical reasons, on the one hand, and practical work, the history of science and technology, and the historical development of the means of deduction used in scientific reasoning, on the other. The theoretical and epistemological context underlines the importance of the principle of sufficient reason as the requirement that the truth of any assertion be sufficiently substantiated (this requirement may be regarded as having various degrees of rigor and completeness depending on the area or object of investigation), that arbitrariness not be permitted in scientific deductions, and that no propositions be taken “on faith” unless they have sufficient reason in the body of present knowledge.