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(Latin dubium, Greek aporí, French doute, German Zweifel ).

A state in which the mind is suspended between two contradictory propositions and unable to assent to either of them. Any number of alternative propositions on the same subject may be in doubt at the same time ; but, strictly speaking, the doubt is attached separately to each one, as between the proposition and its contradictory, i.e. each proposition may or may not be true. Doubt is opposed to 1certitude, or the adhesion of the mind to a proposition without misgiving as to its truth ; and again to opinion, or a mental adhesion to a proposition together with such a misgiving. Doubt is either positive or negative. In the former case, the evidence for and against is so equally balanced as to render decision impossible; in the latter, the doubt arises from the absence of sufficient evidence on either side. It is thus possible that a doubt may be positive on the one side and negative on the other (positivo-negative or negativo-positive), i.e. in cases where evidence on one side only is attainable and does not, of itself, amount to absolute demonstration, as, for instance, in circumstantial evidence. Again, doubt may be either theoretical or practical. The former is concerned with abstract truth and error ; the latter with questions of duty, or of the licitness of actions, or of mere expediency. A further distinction is made between doubt concerning the existence of a particular fact (dubium facti) and doubt in regard to a precept of law (dubium juris). Prudent doubts are distinguished from imprudent, according to the reasonableness or unresonableness of the considerations on which the doubt is based. It should be observed that doubt is a purely subjective condition ; i.e. it belongs only to the mind which has to judge of facts, and has no application to the facts themselves. A proposition or theory which is commonly called doubtful is, therefore, one as to which sufficient evidence to determine assent is not forthcoming; in itself it must be either true or false. Theories which have at one time been regarded as doubtful for want of sufficient evidence, frequently become certainly true or false by reason of the discovery of fresh evidence.

As certitude may be produced either by reason (which deals with evidence) or by faith (which rests on authority), it follows that theoretical doubt may be in like manner concerned with the subject-matter of either reason or faith, that is to say, with philosophy or with religion. Practical doubt is concerned with conduct; and since conduct must be guided by principles afforded by reason or by faith, or by both conjointly, doubt concerning it regards the application of principles already accepted under one or other of the foregoing heads. The resolution of doubt of this kind is the province of moral theology, in regard to questions of right and wrong and in regard to those of mere practical expediency, recourse must be had to the scientific or other principles which properly belong to the subject-matter of the doubt. Thus, for example, doubt as to the actual occurrence of an historical event can only be resolved by consideration of the evidence; doubt as to the doctrine of the sacraments, by ascertaining what is of faith on the subject; doubt as to the morality of a commercial transaction, by the application of the authoritative decisions of moral theology ; while the question of the wisdom or the reverse of the transaction in regard to profit and loss must be determined by commercial knowledge and experience. The legitimacy, or the reverse, of doubt in regard to matters of fact is made evident by the forms of logic (induction and deduction), which, whatever may be the extent of their function as a means of acquiring knowledge, are indispensably necessary as a test of the correctness of conclusions or hypotheses already formed.


The validity of human perception and reasoning in general as guides to objective truth has been frequently called in question. The doubt thus raised has been sometimes of the character called methodic, fictitious, or provisional, and sometimes real, or sceptical, as embodying the conclusion that objective truth cannot be known. Doubt of the former kind is the necessary preliminary to all inquiry, and in this sense philosophy is said by Aristotle (Metaph., III, i) to be "the art of doubting well". Sir W. Hamilton points out (Lect. on Metaphysics, v) that doubt, as a preliminary to philosophical inquiry is the only means by which the necessary removal of prejudice may be effected; as the Baconian method insisted on the primary necessity of putting aside the "idols", or prejudices, by which men's minds are naturally influenced. Thus the Scholastic proof of a proposition or thesis begins by the statement of "doubts", or contrary arguments; after which the evidence for the thesis is given, and finally the doubts are resolved. This, it need hardly be said, is the method pursued in the "Summa" of Saint Thomas Aquinas and still in use in the formal disputations of theological students. An instance of this kind of doubt is the Sic et Non (Yes and No) of Abelard, which consists of a long series of propositions on theological, Scriptural, and philosophical subjects, with a counter-proposition attached to each. The solution of the doubts in the sense of the orthodox thesis, which was clearly intended to follow, was never written; or if so, has not been preserved. (See Victor Cousin's "Fragments Philosophiques".) The philosophical system of Descartes begins with a universal methodic doubt; the famous cogito, ergo sum, on which the whole system is based, is the solution of the philosopher's fundamental doubt of his own existence. This solution had been anticipated by St. Augustine; who took the subjective certainty of one's own existence as the ground of all certainty, e.g.,

Tu, qui vis te nosse, scis esse te? Scio. Unde scis? Nescio. Cogitare te scis? Scio. (Sol., II, i);

Utrum aëris sit vis vivendi, an ignis, dubitaverunt homines; vivere se tamen et meminisse et intelligere et velle et cogitare et scire et judicare quis dubitet? Quandoquidem etiam si dubitet vivit; si dubitat, dubitare se intelligit, etc. (De Trin., X, xiv).

In general it may be said that doubt, either expressed or implied, is involved in all intellectual research.

Among the systems in which doubt as to the trustworthiness of human faculties is not merely provisionally assumed, but is genuine and final, those which find in a supernatural revelation the guide to truth which natural reason fails to provide must be distinguished from those which hold doubt to be the final conclusion of all inquiry into truth. The former depreciate reason in the interests of faith ; the latter take reason as the only possible guide, but find no ground for confidence in it. To the former class belongs Nicholas of Cusa (1440), who was the author of two sceptical treatises on human knowledge ; certainty is to be found, according to his view, only through the mystical knowledge of God. The scepticism of Montaigne made a reservation (whether sincerely or not is uncertain) in favour of revealed truth ; and the same principle was advocated by Charron, Sanchez, and Le Vayer. Hume, in his sceptical essays on miracles and immortality, also attributed a final authority to revelation ; but with obvious insincerity. The sceptical views of Hobbes, combined with his peculiar theory of government, made all conviction, including that of religious truth, dependent on the civil authority. Glanvill's "The Vanity of Dogmatizing", or "Scepsis Scientifica", grounded a serious defense of revealed religion on the uncertainty of natural knowledge. Balfour's "Defense of Philosophic Doubt", based on the indemonstrability of ultimate truths, is an attempt in the same direction. (See FIDEISM.)

In the second class are to be reckoned the various systems of genuine skepticism. This appeared in Greek philosophy at a very early date. Heraclitus held the senses to be untrustworthy (kakoí mártures) and misleading, though he also conceived a supersensuous knowledge of the universal reason, immanent in the cosmos, to be attainable. Zeno of Elea defended the doctrine of the unity and permanence of being by propounding a series of "hypotheses", each of which resulted in a contradiction, and by means of them sought to demonstrate the unreality of the manifold and changing. The subjective principle of the Sophists (Protagoras, Gorgias, and others of less note) that "man is the measure of all things" implies doubt, or skepticism, as to all objective reality. Knowledge is resolved by Protagoras into mere variable opinion; and Gorgias asserts that nothing really exists, that if anything existed, it could not be known, and that if such knowledge were possible it would be incommunicable. The Pyrrhonists, or Sceptics, held everything in doubt, even the fact of doubting. The Middle Academics, whose chief representatives were Arcesilaus and Carneades, while doubting all knowledge, held, nevertheless, that probability could be recognized in varying degrees. The "Encyclopedia" of Diderot and d'Alembert comments on the odd self-contradiction of Montaigne, who claimed a higher degree of probability for the Pyrrhonist than for the Academic opinion. Sextus Empiricus advanced the theory, often since maintained, that the syllogism is really a petitio principii, and that demonstration is therefore impossible. Bayle, in his celebrated "Dictionary", subjected the philosophy of his time to severe destructive criticism, but was confessedly unable to supply its deficiencies. Hume's position was purely negative for him, neither the existence of the external world nor that of the mind by which it is known was capable of demonstration; and the conclusion of Kant's "Critique of Pure Reason", that the "thing in itself" (Ding an sich) is unknowable though certainly existing, is evidently sceptical (though the author himself rejected the title), since it embodies a purely negative doubt as to the nature of "transcendent" reality. Kant's argument for the existence of God, as rationally indemonstrable, but postulated by the practical reason, necessarily results in a very limited conception of the Divine nature. Lamennais made general consent, or the common sense of mankind, the only ground of certitude; the individual reason he held to be incapable of attaining it. "Nothing is so evident to us today that we can be sure we shall not find it either doubtful or erroneous to-morrow" (Essai sur l'indifférence, II, xiii).

It may be observed that theories which deny the validity of simple experience as a guide to truth are really instances of doubt, because, though they assert dogmatically the inadequacy of widely accepted evidence, they are nevertheless in that state of suspense by which doubt is properly characterized in regard to the reality commonly held to be made known by experience. Thus the mental attitude which received from Professor Huxley the name of Agnosticism is a strictly doubtful one towards all that lies beyond sense-experience. The doubt is purely negative in this view; whatever is not cognizable by the aid of the senses is held to be unknowable; God may exist, or He may not, but we can neither affirm His existence nor deny it. Again, the system or method known as Pragmatism regards all reality as doubtful; truth is the correspondence of ideas with one another, and cannot be regarded as anything final, but must perpetually change with the progress of human thought; knowledge must be taken at its "face value" from moment to moment, as a practical guide to well-being, and must not be regarded as having any necessary correspondence with definite and permanent reality.


In regard to relgion, doubt has at different times assumed a variety of forms. It is perhaps uncertain how far the ancient mythologies received or even demanded exact belief ; it is at any rate certain that they were, as a rule, not considered worthy of serious attention by the philosophers of any school. The atheism which formed part of the charge on which Socrates was condemned was an offense against the State rather than against religion in itself (see Lecky, Hist. of European Morals, ii). The faith demanded by the Christian Revelation stands on a different footing from the belief claimed by any other religion. Since it rests on divine authority, it implies an obligation to believe on the part of all to whom it is proposed; and faith being an act of the will as well as of the intellect, its refusal involves not merely intellectual error, but also some degree of moral perversity. It follows that doubt in regard to the Christian religion is equivalent to its total rejection, the ground of its acceptance being necessarily in every case the authority on which it is proposed, and not, as with philosophical or scientific doctrines, its intrinsic demonstrability in detail. Thus, whereas a philosophical or scientific opinion may be held provisionally and subject to an unresolved doubt, no such position can be held towards the doctrines of Christianity ; their authority must be either accepted or rejected. The unconditional, interior assent which the Church demands to the Divine authority of revelation is incompatible with any doubt as to its validity. Gregory XVI, by the Brief "Dum acerbissimas", 26 Sept., 1835, condemned the teaching of Hermes that all theological inquiry should be based on positive doubt ( Denzinger, 10th ed., no. 1619); and the Vatican Council declared (Sess. II, ch. sxxi):

"revelata vera esse credimus, non propter intrinsecam rerum veritatem naturali rationis lumine perspectam, sed propter auctoritatem ipsius Dei revelantis, qui nec falli nec fallere potest", i.e. we believe the things that are revealed to be true, not because of an intrinsic truth which reason perceives, but because of the authority of God Who is the Author of Revelation, and Who can neither deceive nor be deceived.

Heresies have, however, generally had the character rather of dogmatic assertion than of mere doubt, though they arose from a more or less prevalent state of doubt as to doctrines imperfectly understood or not yet authoritatively defined. The devotion to classical studies which followed upon the fall of Constantinople in 1463 and the dispersion of its literary treasures gave rise to the humanism, or literary revival, of the Renaissance, and in many cases resulted in a skeptical attitude towards religion. This skepticism, however, was by no means universal among the Humanists, and was due rather to lack of interest in theological as compared with literary and philosophical, study, than to any reasoned criticism of religious doctrine. (See Pastor, "History of the Popes ", chapters on the Renaissance.) It helped to prepare the way however, for the Reformation which, beginning with a revolt against ecclesiastical authority, called all the doctrines of Christianity in question, rejecting those which failed to gain the approval of the different leaders of the movement. Thus among Protestants in general there is great variety of opinion on religious doctrines, those which are firmly held by some being considered doubtful by others, and by others again, rejected as false. Anglicanism, especially, leaves open many of the tenets which the Catholic Church holds as of faith, and thus endeavours to comprehend within its boundaries persons who differ widely from one another on important subjects. The Catholic Church, on the other hand, pronounces authoritatively as to the truth or falsehood of opinions, by means of general councils , professions of faith, infallible decisions of the supreme pontiff, and the ordinary teaching of her Doctors. As St. Avitus , in the sixth century, declared, "it is the law of the councils that if any doubt have arisen in matters which regard the state of the Church, we are to have recourse to the chief priest of the Roman Church " (Ep. xxxvi in P. L., LIX, 253). Doubt as to the Faith is thus impossible in the Catholic Church without infringing the principle of authority on which the Church itself depends. The field, however, which is open to a variety of opinions on questions not directly involving the essential doctrines of the Faith is still a very wide one; and though its extent may be further limited by future dogmatic decisions, it is probable on the other hand that it will be increased in the future, as in the past, by the emergence of doubtful questions as to the exact bearing of dogmatic truth upon fresh discoveries or theories of all kinds.

It will be evident from what has been said that doubt cannot coexist either with faith or knowledge in regard to any given subject; faith and doubt are mutually exclusive, and knowledge which is limited by a doubt, becomes, in regard to the subject or part of a subject to which the doubt applies, no longer knowledge but opinion. A moral certainty --that is, one which is founded on the normal course of human action--does not strictly exclude doubt, but, as excluding prudent doubt, must be considered a sufficient practical guide (cf. Butler, "Analogy of Religion", introduction, and pt. II, ch. vi). Thus doubt is sometimes said to imply belief ; though such belief or practical certainty cannot properly be held to rise above the most probable kind of opinion. The rhetorical conception of the faith that "lives in honest doubt" (Tennyson, In Memoriam) must be taken to signify that truthful and serious habit of mind which refuses to submit to deception on motives furnished by intellectual sloth or the desire of worldly advantage. Catholic philosophy is entirely opposed both to the Pyrrhonist doubt of external reality and to that form of Idealism which is closely connected with the Kantian method on its sceptical side, and which seeks to reduce all dogma to the mere expression of subjective religious conceptions, relegating the objective facts with which dogma is concerned to the domain of symbol and parable. In the view of the Scholastic system human experience is a true perception of external reality through the senses and the intellect ; phenomena being the object both of the senses, which they directly affect, and, after a different manner, of the intellect, which apprehends through sensible impressions the true nature and principles of the reality which causes those impressions. The facts of revelation to which the Church bears witness are in this sense real and objective, and may neither be explained away nor set aside by any system of historical or scientific criticism. Such is the purport of the Encyclical "Pascendi Dominici gregis" (1907), which both controverts and condemns the attempt to evacuate dogma of its true significance made by the method of religious speculation known as Modernism.


Practical doubt, or doubt as to the lawfulness of an action is, according to the teaching of moral theology , incompatible with right action; since to act with a doubtful conscience is obviously to act in disregard of the moral law. To act with a doubtful conscience is therefore, sinful ; and the doubt must be removed before any action can be justified. It frequently happens, however, that the solution of a practical doubt is not attainable, while some decision is necessary. In such cases the conscience may obtain a "reflexive" certainty by adopting an approved opinion as to the lawfulness of the action contemplated, apart from the intrinsic merits of the question. The question has been much discussed among different schools of theologians whether the opinion so followed must be of greatly preponderating authority in favour of liberty in order to justify an action the lawfulness of which appears intrinsically doubtful, whether it must be merely more probable than the contrary one, or equally probable, or merely probable in itself, even though less so than its contrary. (See MORAL THEOLOGY; PROBABILISM.) The fast, however, is the theory now generally accepted for all practical purposes; and the principle that lex dubia non obligat --i.e. that a law which is doubtful in its application to the case in hand does not bind--is universally admitted. It must be observed, however, that where the question is one not merely of positive law but of securing a certain practical result, only the "safer" course may be followed. No opinion however probable, is allowed to take precedence of the most certain means of securing such ends; e.g. in providing for the validity of the sacraments, in discharging obligations of justice, or in avoiding injury to others. Thus doubtful baptisms and ordinations must be repeated conditionally. (See AGNOSTICISM; CERTITUDE; EPISTEMOLOGY ; FAITH ; HERESY; INFALLIBILITY ; SCEPTICISM.)

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