193. But let us see if it is necessary to change the formula by which the principle of contradiction has hitherto been expressed. The first observation of Kant refers to the word _impossible_, which he considers unnecessarily added, since the apodictic certainty, which we wish to express, should be contained in the proposition itself. Kant's formula of the principle is this: "a predicate which is _opposed_ to a subject, does not belong to it." What is the meaning of the word _impossible_? "Possible and impossible absolutely, are said in relation to the terms. Possible, because the predicate is not opposed to the subject; impossible, because the predicate is opposed to the subject;" says St. Thomas,[20] and with him agree all the schools. Therefore, impossibility is the opposition of the predicate to the subject, and to be repugnant is the same thing as to be impossible, and Kant uses the very language which he blames in others. The common formula might be expressed in this manner: "there is opposition in the same thing being and not-being at the same time," or, "being is opposed to not-being," or, "being excludes not-being," or, "every thing is equal to itself;" and Kant expresses nothing more when he says: "a predicate which is opposed to a subject does not belong to it." [20] P. 1^a, Q. 25^a, A. 3. 194. As a universal criterion, there is more exactness in the common formula than in that of Kant. The latter restricts the principle to the relation of predicate and subject, and consequently to the purely ideal order, making it of no value for the real, unless by a sort of enlargement. This enlargement, although legitimate and easy, is not needed in the common formula: by saying being excludes not-being, we embrace the ideal and the real, and present to the mind the impossibility, not only of contradictory judgments, but also of contradictory things.