Necessity, in a general way, denotes a strict connection between different beings, or the different elements of a being, or between a being and its existence. It is therefore a primary and fundamental notion, and it is important to determine its various meanings and applications in philosophy and theology.

In Logic, the Schoolmen, studying the mutual relations of concepts which form the matter of our judgments, divided the judgments or propositions into judgments in necessary matter ( in materia necessaria ), and judgments in contingent matter ( in materia contingenti ). (Cf. S. Thom., I Perihermen, lect. xiii.) The judgments in necessary matter were known as propositiones per se ; they are called by modern philosophers "analytic", "rational", "pure", or "a priori" judgments. The propositio per se is defined by the Schoolmen as one the predicate of which is either a constitutive element or a natural property of the subject. Such is the case with primary truths, metaphysical, and mathematical principles. (Cf. S. Thom., "in I Anal.", lect. x and xxxv; "de Anima", II, lect. xiv.) It is by ignoring the last part of this definition and arbitrarily restricting the concept of analytic judgments to those of which the predicate is a constitutive element of the subject, that Kant invented the false notion of synthetic- a priori judgments.

Considered under its metaphysical aspect, being in its relation to existence is dividend into necessary and contingent. A necessary being is one of which the existence is included in and identical with its very essence. The different beings which we observe in our daily experience are subject to beginning, to change, to perfection, and to destruction; existence is not essential to them and they have not in themselves the reason of their existence ; they are contingent. Their existence comes to them from an external efficient cause. It is from the real existence of contingent beings that we arrive at the notion and prove the existence of a necessary being-one that produces them but is not produced, one whose existence is its own essence and nature, that is at the same time eternal, all-perfect, infinite, viz., God (see CONTINGENCY). And so in relation to existence, God alone is absolutely necessary, all others are contingent.

When we consider the divers beings, not from the point of view of existence, but in relation to their constitution and activity, necessity may be classified as metaphysical, physical , and moral.

• Metaphysical necessity implies that a thing is what it is, viz., it has the elements essential to its specific nature. It is a metaphysical necessity for God to be infinite, man rational, an animal a living being. Metaphysical necessity is absolute.
• Physical necessity exists in connection with the activity of the material beings which constitute the universe. While they are contingent as to their existence, contingent also as to their actual relations (for God could have created another order than the present one), they are, however, necessarily determined in their activity, both as to its exercises and its specific character. But this determination is dependent upon certain conditions, the presence of which is required, the absence of one or the other of them preventing altogether the exercise or normal exercise of this activity. The laws of nature should always be understood with that limitation: all conditions being realized. The laws of nature, therefore, being subject to physical necessity are neither absolutely necessary, as materialistic Mechanism asserts, nor merely contingent, as the partisans of the philosophy of contingency declare; but they are conditionally or hypothetically necessary. This hypothetical necessity is also called by some consequent necessity.
• Moral necessity is necessity as applied to the activity of free beings. We know that men under certain circumstances, although they are free, will act in such and such a way. It is morally necessary that such a man in such circumstances act honestly; it is morally necessary that several historians, relating certain facts, should tell the truth concerning them. This moral necessity is the basis of moral certitude in historical and moral sciences. The term is also used with reference to freedom of the will to denote any undue physical or moral influence that might prevent the will from freely choosing to act or not act, to choose one thing in preference to another. The derivatives, necessitation and necessarianism, in their philosophical signification express the doctrine that the will in all its activity is invariably determined by physical or psychical antecedent conditions (see DETERMINISM; FREE WILL).

In theology the notion of necessity is sometimes applied with special meaning. Theologians divide necessity into absolute and moral. A thing is said to be absolutely necessary when without it a certain end cannot possibly be reached. Thus revelation is absolutely necessary for man to know the mysteries of faith, and grace to perform any supernatural act. Something is said to be morally necessary when a certain end could, absolutely speaking, be reached without it, but cannot actually and properly be reached without it, under existing conditions. Thus, we may say that, absolutely speaking, man as such is able to know all the truths of the natural order or to observe all the precepts of the natural law ; but considering the concrete circumstances of human life in the present order, men as a whole cannot actually do so without revelation or grace. Revelation and grace are morally necessary to man to know sufficiently all the truths of the natural law (cf. Summa Theologica, I:1:1 ; "Contra Gentil.", I, iv).

Again, in relation to the means necessary to salvation theologians divide necessity into necessity of means and necessity of precept. In the first case the means is so necessary to salvation that without it (absolute necessity) or its substitute (relative necessity), even if the omission is guiltless, the end cannot be reached. Thus faith and baptism of water are necessary by a necessity of means, the former absolutely, the latter relatively, for salvation. In the second case, necessity is based on a positive precept, commanding something the omission of which, unless culpable, does not absolutely prevent the reaching of the end.

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