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Necessity signifies something fixed or determined that must be, or be so, and cannot be otherwise. Man cannot think that to be is the same as not to be, nor can he at once both affirm and deny the same of the same. Thus some awareness of necessity is included in the first principles of human thought. However, a more distinct knowledge of necessity and its diverse kinds is attained with the notion of causality. A cause is something that influences the being of another, or upon which something must follow with dependence in being. To know in the full sense of genuine understanding is, by common consent, to know the proper cause or necessary reason of being, on account of which something is, or is so, and cannot be otherwise.

When necessity is considered in regard to being or that which is, it is opposed to contingency or corruptibility. A changeable thing, as such, is not a necessary being. Considered in relation to knowledge, necessity is opposed to opinion or probability, whereas in action it is opposed to freedom.

Origins in Aristotle. In order to explain the meaning of necessity, aristotle lists examples according to the different kinds of causes (Meta. 1015a 201015b 16). Something may be necessary as a concurrent cause of being and life, as respiration is necessary for an organism. Furthermore, something may be necessary for the attaining of a good or the avoiding of an evil, as a journey may be necessary, or the taking of medicine. Again, force or violence have necessity, and also whatever is effected by force or violence. In general, that is necessary which cannot be otherwise, whether by reason of an intrinsic cause, such as matter or form or intrinsic nature, or by reason of an extrinsic cause, whether final or efficient. Just as in logical demonstrations necessary conclusions follow from necessary premises, so that which is necessary may be either an effect having a necessary cause, or a cause that is necessary of itself and not dependent upon another cause. In ancient times the heavenly bodies were thought to be necessary beings of incorruptible nature, yet dependent upon another cause. The first cause of all is itself uncaused. This is the strictly necessary being, the one that is purely actual and immutable.

Scholastic Doctrine. Considerations such as these, together with many others, were elaborated in the works of the medieval scholastics, among which the teaching of St. thomas aquinas is most representative. They taught that God is the only necessary being not dependent upon any other cause. God is subsistent being and intelligence. In Him being and essence, or nature, are identical, and so He is, or exists, with the most absolute necessity (Summa theologiae 1a, 3.4).

Necessity in Creatures. Furthermore, the scholastics taught that God created the world of bodies and intelligent spirits by the free exercise of His omnipotence, not from eternity but at the beginning of time, and not by necessity of His nature, or out of need for them, but out of generosity and benevolence (ST 1a, 19.3; 44.1). Hence the world is contingent upon God's good pleasure, and constantly depends in being on the Creator, without whose conserving act it would cease to be. Nevertheless, even in contingent beings there is something necessary, and there is nothing so contingent that it does not have necessary aspects (ST 1a, 86.3). Speaking of creatures as they are according to their own being and natures, spiritual beings together with their essential properties are strictly necessary and cannot be otherwise by any power within themselves. Material beings also are strictly necessary as regards matter and motion in general. Although particular bodies are contingent and corruptible, still in the course of nature they do not come from nothing, nor do they pass into nothing, but the corruption of one is the generation of another. Matter as the primary subject of change can be neither generated nor corrupted, nor can the general principles of motion cease to operate by any defect in themselves. The course of nature is neither chaotic nor perfectly regular; it includes many kinds of events and products that occur with regularity, as well as many that are incidental and accidental. Events and products that occur with regularity have determined efficient and material causes, and they share the necessity of their causes. Incidental and accidental occurrences, although undetermined or casual in particular, are necessary concomitants in the world order. (Cf. C. gent. 2.30.)

Natural Necessity. Because the course of nature is not perfectly regular, and because in particular cases the materials required for a process might be lacking or indisposed, or the agent might be prevented from producing its regular effect, the question was raised about the possibility of a philosophy or science of nature (see philosophy of nature). Do natural things have principles, causes, or elements that can be discovered and by which they can be explained scientifically? This question was answered in the affirmative from two points of view. In the first place, it was thought that the various species of natural things are distinguishable empirically by differences that are distinct and irreducible and that occur with sufficient regularity to manifest the definable natures with their consequent properties. These natures are changeable and corruptible as they are found in individuals, but when understood abstractly and according to their essential principles, they are necessary and universal. In the second place, the orderly processes of nature attain great natural advantages that are regularly produced and that are the ends or goals of natural activity. If one supposes that the natural end which is usually attained will in fact be attained, then certain other things are necessary, namely, certain materials to be determined or actualized by certain agents. To this there is no exception, and therefore it was maintained that from this point of view natural science is possible. It was thought that man can attain knowledge of natural things that is necessary and universal, not merely in regard to the general aspects of nature, but also in regard to the distinct species with their parts and interrelations. In order to attain a more detailed knowledge of nature, the scholastics undertook some experimentation and measurement, but progress was slow and few appreciated the importance of quantitative considerations. What was more characteristic of their thought was that while acknowledging that matter and motion are necessary, they sought a reason for this necessity in the end or final cause, which they admitted to be only conditionally or hypothetically necessary. Their cardinal point was that if the end is, or is to be, then the antecedents must be, without exception, in the order of nature. (Cf. In 2 phys. 15.)

Mathematical Necessity. Necessity in the objects of mathematics was held to be clearer and stronger than that in physical things (see mathematics, philosophy of). The mathematician abstracted from sensible matter and motion, and considered quantitative beings merely as they are imaginable and intelligible. Mathematical numbers and figures were considered as existing only in the mind of the mathematician, and were constructed in the mind out of their known principles and elements. Hence they were regarded as more clearly and certainly known than physical things, and it was thought that many of their properties could be demonstrated as necessary. For many centuries mathematics was regarded as the paradigm of necessary and universal knowledge. (Cf. In Boeth. de Trin. 5.2; 6.1.)

However, the scholastics did not admit that in either physics or mathematics one considers forms or essences absolutely, according to their strictest necessity. This is the business of the metaphysician, who relates effects to their ultimate causes, whether of being or of truth and goodness, and tries to explain all things in relation to their strictly necessary cause, namely, God. (see metaphysics.)

Moral Necessity. The scholastics pointed out a likeness between the necessity found in physical processes and that found in moral actions (see ethics). In a physical process there is unconditioned or absolute necessity on the part of the matter and the agent, and conditioned or hypothetical necessity on the part of the goal that is or is to be attained by determined means. So also in moral action there is absolute necessity in the principle that each man desires to be humanly happy, and hypothetical necessity to choose the reasonable good and avoid evil in order to achieve genuine happiness. The desire for good is naturally determined, not free, but together with reason it is the principle of free choice of the means to happiness. Yet freedom of choice might be impeded or lessened by ignorance and emotion. (Cf. In 6 eth. 3.114252.)

Logical Necessity. Necessity was admitted also in the purely logical order of mental operations. The mind begins to function and so must first apprehend its own object, called being, and then something opposed to being, which might be nonbeing or this as opposed to that. Then with natural necessity man judges that being is not nonbeing, or this is not that. Thus he attains the first principle of thought, called the principle of contradiction, which is not a supposition but an axiom, that is, a necessary, self-grounded, or self-evident principle. Likewise, after one knows whole and part, he must judge with natural necessity that the whole is greater than the part. In regard to these primitive concepts and principles the mind is naturally determined by the clearest evidence, and so error is here impossible. From principles that are known to be true and necessary, one can by valid reasoning draw conclusions that are also true and necessary (see demonstration). In formal logic it is sufficient that the consequence or logical connection between the principles and the conclusion be valid and necessary, according to the laws of reasoning based on the axioms or postulates of the system. The necessity in this case is not absolute but hypothetical: if a valid conclusion is to be reached, the premises must be thus or so; and if the premises are thus or so, this conclusion must follow because it is the only one permitted by the axioms, all others being excluded. (Cf. In 1 anal. post. 13; In 2 anal. post. 7.)

Nonscholastic Thought. During the scholastic period there were many thinkers, both Platonists and nominalists, who rejected the moderate realism of Aristotle and his medieval followers. Nominalists emphasized the contingencies in sensory experience, and neglected or denied the intelligible necessities of being, with its necessary reasons and causes. Platonists did not look for intelligibility in the sensible world, but rather in the world of transcendent ideas and spiritual realities. (see nominalism; platonism.)

Cartesianism. At the beginning of the modern period, R. descartes endeavored to make a complete break from the methods and principles of ordinary thought and traditional philosophy. He chose to proceed not from knowledge of something that is necessary and universal, such as the principle of contradiction, but from the particular fact of his own thought, which he identified with his own being. Thereafter, he went step by step from one clear and distinct idea to another, without seeking in all cases a rational or intelligible connection between his ideas.

The method and teachings of Descartes resulted in opposing tendencies of rationalism and empiricism, and of idealism and materialism, with attempts to unite both tendencies in various forms of monism.

Rationalism and Empiricism. B. spinoza and G. W. leibniz developed the rationalist tendencies into a deterministic view of God and the world. According to these thinkers, necessity was opposed to freedom only when it resulted from external domination or compulsion, not from internal determination. God is free because He is self-determined, and all nature is determined by God. Hence everything is necessary and nothing contingent, nor is there genuine freedom of choice in God or man.

T. hobbes, J. locke, and D. hume also defined freedom as lack of external compulsion. Hume maintained that everything has a cause, and denied chance, but held that man does not know necessary causes in nature. Kant saw that this restriction of human knowledge to phenomena threatened the validity of physical science as developed particularly by I. Newton. Hence, in order to defend this kind of science, Kant attributed the elements of necessity and universality to the structure of the mind that knows, rather than to the thing known. He held that the mind has necessary ways of knowing, antecedent to all experience of particular things, and maintained that the essences of things and their necessary causes are speculatively unknowable. However, he admitted that the acknowledgement of God and freedom, morality, and immortality are practically necessary for a good life.

Idealism and Materialism. This strain of subjectivism was further developed into idealism by J. G. fichte,F. schelling, and G. W. F. hegel to the point where the inner necessity of the idea was identified with the outer necessity of historical fact. Communists now interpret the Hegelian dialectic as a determined order of materialistic evolution that eventually and inevitably will favor themselves.

Formalism. Many contemporary logicians and mathematicians profess to have no interest in principles that are true and necessary. They employ terms that are defined only by the postulates of the axiomatic system they freely invent, and frankly acknowledge that one cannot know whether the systems in actual use are either complete or self-consistent.

Critique. This brief account shows that modern ways of philosophical thought are far removed from the natural realism of Aristotle and the scholastics. Nevertheless, universality and consistency remain the goals of thought, and these necessarily exclude self-contradiction. It appears impossible to doubt the necessity of the principle of contradiction, or the ability of the human mind to know truth, and to discover in some cases, at least, the necessary reasons and causes of being, without which things cannot be as they are or as they ought to be. In such knowledge of the necessary reasons and causes of being, genuine science, philosophy, and wisdom are commonly thought to consist.

See Also: contingency; possibility.

Bibliography: g. jalbert, Nécessité et contingence chez saint Thomas d'Aquin et chez ses prédécesseurs (Ottawa 1961). j. chevalier, La Notion du nécessaire chez Aristote et chez ses prédécesseurs (Paris 1915). j. maritain, "Réflexions sur la nécessité et la contingence," Angelicum 14 (1937) 281295. m. j. adler, ed., The Great Ideas: A Syntopicon of Great Books of the Western World, 2 v. (Chicago 1952); v.2, 3 of Great Books of the Western World 2:251269. a. guzzo and v. mathieu, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 3:828837. r. eisler, Wörterbuch der philosophischen Begriffe, 3 v. (4th ed. Berlin 192730) 2:259271.

[w. h. kane]

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