The objects of human thought and inquiry are either relations of ideas or matters of fact. To the former class belong the objects of mathematics, the truths of which, since they are analytic (i. e., merely explicate in the predicate the characteristics already contained in the subject, and add nothing new to this), and since they concern possible relations only, not reality, possess intuitive or demonstrative certainty. It is only propositions concerning quantity and number that are discoverable a priori by the mere operation of thought, without dependence on real existence, and that can be proved from the impossibility of their opposites—mathematics is the only demonstrative science.
We reach certainty in matters of fact by direct perception, or by inferences from other facts, when they transcend the testimony of our senses and memory. These arguments from experience are of an entirely different sort from the rational demonstrations of mathematics; as the contrary of a fact is always thinkable (the proposition that the sun will not rise to-morrow implies no logical contradiction), they yield, strictly speaking, probability only, no matter how strong our conviction of their accuracy may be. Nevertheless it is advisable to separate this species of inferences from experience—whose certainty is not doubted except by the philosophers—from uncertain probabilities, as a class intermediate between the latter and demonstrative truth (demonstrations—proofs—probabilities). All reasonings concerning matters of fact are based on the relation of cause and effect. Whence, then, do we obtain the knowledge of cause and effect? Not by a priori thought. Pure reason is able only to analyze concepts into their elements, not to connect new predicates with them. All its judgments are analytic, while synthetic judgments rest on experience. Judgments concerning causation belong in this latter class, for effects are entirely distinct from causes; the effect is not contained in the cause, nor the latter in the former. In the case of a phenomenon previously unknown we cannot tell from what causes it has proceeded, nor what its effect will be. We argue that fire will warm us, and bread afford nourishment, because we have often perceived these causal pairs closely connected in space and time. But even experience does not vouchsafe all that we desire. It shows nothing more than the coexistence and succession of phenomena and events; while the judgment itself, e. g., that the motion of one body stands in causal connection with that of another, asserts more than mere contiguity in space and time, it affirms not merely that the one precedes the other, but that it produces it—not merely that the second follows the first, but that it results from it. The bond which connects the two events, the force that puts forth the second from the first, the necessary connection between the two is not perceived, but added to perception by thought, construed into it.[1] What, then, is the occasion and what the warrant for transforming perceived succession in time into causal succession, for substituting must for is, for interpreting the observed connection of fact into a necessary connection which always eludes observation?
[Footnote 1: The weakness of the concept of cause had been recognized before Hume by the skeptic, J. Glanvil (1636-80). Causality itself cannot be perceived; we infer it from the constant succession of two phenomena, without being able to show warrant for the transformation of thereafter into thereby.]
We do not causally connect every chance pair of successive events, but those only which have been repeatedly observed together. The wonder is, then, that through oft-repeated observation of certain objects we come to believe that we know something about the behavior of other like objects, and the further behavior of these same ones. From the fact that I have seen a given apple fall ten times to the ground, I infer that all the apples in the world do the same when loosened, instead of flying upward, which, in itself, is quite as thinkable; I infer further that this has always been the case, and will continue to be so to all eternity. Where is the intermediate link between the proposition, "I have found that such an object has always been attended with such an effect," and this other, "I foresee that other objects which are, in appearance, similar, will be attended with similar effects"? This postulate, that the future will be like the past, and that like causes will have like effects, rests on a purely psychological basis. In virtue of the laws of association the sight of an object or event vividly recalls the image of a second, often observed in connection with the former, and leads us involuntarily to expect its appearance anew. The idea of causal connection is based on feeling (the feeling of inner determination to pass from one idea to a second), not upon insight; it is a product of the imagination, not of the understanding. From the habitual perception of two events in connection (sunshine and heat) arises the mental determination to think of the second when we perceive the first, and, anticipating the senses, to count on its appearance. It is now possible to state of what impression the idea of the causal nexus is the copy: the impression on which it is based is the habitual transition from the idea of a thing to its customary attendant. Hence the idea of causality has a purely subjective significance, not the objective one which we ascribe to it. It is impossible to determine whether there is a real necessity of becoming corresponding to the felt necessity of thought. In life we never doubt the fact, but for science our conviction of the uniformity of nature remains a merely probable (though a very highly probable) conviction. Complete certainty is vouchsafed only by rational demonstration and immediate experience. The necessary bond which we postulate between cause and effect can neither be demonstrated nor felt.
If all experiential reasonings depend on the idea of causality, and this has no other support than subjective mental habit, it follows that all knowledge of nature which goes beyond mere observed fact is not knowledge (neither demonstrative knowledge nor knowledge of fact), but belief.[1] The probability of our belief in the regularity of natural phenomena increases, indeed, with every new verification of the assumptions based thereon; but, as has been shown, it never rises to absolute certainty. Nevertheless inferences from experience are trustworthy and entirely sufficient for practical life, and the aim of the above skeptical deliverances was not to shake belief—only a fool or a lunatic can doubt in earnest the immutability of nature—but only to make it clear that it is mere belief, and not, as hitherto held, demonstrative or factual knowledge. Our doubt is intended to define the boundary between knowledge and belief, and to destroy that absolute confidence which is a hindrance rather than a help to investigation. We should recognize it as a wise provision of nature that the regulation of our thoughts and the belief in the objective validity of our anticipation of future events have not been confided to the weak, inconstant, inert, and fallacious reason, but to a powerful instinct. In life and action we are governed by this natural impulse, in spite of all the scruples of the skeptical reason.
[Footnote 1: Hume distinguishes belief as a form of knowledge from religious faith, both in fact and in name. In the Treatise—the passage is wanting in the Enquiry—our conviction of the external existence of the objects of perception is also ascribed to the former, which later formed Jacobi's point of departure. Religious faith is referred to revelation.]
In Hume's earlier work his destructive critique of the idea of cause is accompanied by a deliverance in a similar strain on the concept of substance, which is not included in the shorter revision. Substances are not perceived through impressions, but only qualities and powers. The unknown something which is supposed to have qualities, or in which these are supposed to inhere, is an unnecessary fiction of the imagination. A permanent similarity of attributes by no means requires a self-identical support for these. A thing is nothing more than a collection of qualities, to which we give a special name because they are always found together. The idea of substance, like the idea of cause, is founded in a subjective habit which we erroneously objectify. The impression from which it has arisen is our inner perception that our thought remains constant in the repeated experience of the same group of qualities (whenever I see sugar, I do the same thing, that is, I combine the qualities white color, sweet taste, hardness, etc., with one another), or the impression of a uniform combination of ideas. The idea of substance becomes erroneous through the fact that we refer it not to the inner activity of representation, to which it rightly belongs, but to the external group of qualities, and make it a real, permanent substratum for the latter. Mental substances disappear along with material substances. The soul or mind is, in reality, nothing more than the sum of our inner states, a collection of ideas which flow on in a continuous and regular stream; it is like a stage, across which feelings, perceptions, thoughts, and volitions are passing while it does not itself come into sight. A permanent self or ego, as a substratum of ideas, is not perceived; there is no invariable, permanent impression. That which leads to the assumption of personal identity is only the frequent repetition of similar trains of ideas, and the gradual succession of our ideas, which is easily confused with constancy. Thus robbed of its substantiality, the soul has no further claims to immateriality and immortality, and suicide ceases to be a crime.[1]
[Footnote 1: Cf. the essays on Suicide and the Immortality of the Soul, 1783, whose authorship by Hume, however, is not absolutely established [of. Green and Grose, as above, p. 221, note first.—TR.]
Is Hume roundly to be called a skeptic? [1] He never impugned the validity of mathematical reasonings, nor experimental truths concerning matters of fact; in regard to the former his thought is rationalistic, in regard to the latter it is empirical or, more accurately, sensationalistic. His attitude toward the empirical sciences of nature and of mind is that of a semi-skeptic or probabilist, in so far as they go beyond the establishment of facts to the proof of connections under law and to inferences concerning the future. Habit is for him a safe guide for life, although it does not go beyond probabilities; absolute knowledge is unattainable for us, but not indispensable. Toward metaphysics, as an alleged science of the suprasensible, he takes up an entirely negative attitude. If an argument from experience is to be assured of merely that degree of probability which is sufficient for belief, it must not only have a well-established fact (an impression or memory-image) for its starting point, but, together with its conclusion, it must keep within the limits of possible experience. The limits of possible experience are also the limits of the knowable; inferences to the continued existence of the soul after death and to the being of God are vain sophistry and illusion. According to the famous conclusion of the Essay, all volumes which contain anything other than "abstract reasonings concerning quantity or number" or "experimental reasonings concerning matter of fact and existence" deserve to be committed to the flames. In view of this limitation of knowledge to that which is capable of exact measurement and that which is present in experience, as well of the principle that the elements added by thought are to be sharply distinguished from the positively given (the immediate facts of perception), we must agree with those who call Hume the father of modern positivism.[2]
[Footnote 1: In the Essay, Hume describes his own standpoint as mitigated or academical skepticism in antithesis to the Cartesian, which from doubt and through doubt hopes to reach the indubitable, and to the excessive skepticism of Pyrrhonism, which cripples the impulse to inquiry. This moderate skepticism asks us only, after resisting the tendency to unreflecting conclusions, to make a duty of deliberation and caution in judging, and to restrain inquiry within those fields which are accessible to our knowledge, i.e., the fields of mathematics and empirical fact. In the Treatise Hume had favored a sharper skepticism and extended his doubt more widely, e.g., even to the trustworthiness of geometry. Cf. on this point Ed. Grimm, Zur Geschichte des Erkenntnissproblems, 1890, p, 559 seq.]