The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume X. The Age of Johnson.
§ 5. Hume’s “New Medium”
Hume is quite clear as to the method of his enquiry. He recognised that Locke and others had anticipated him in the “attempt to introduce the experimental method of reasoning into moral subjects.” Locke had, also, opened the way for deriving a system of philosophy from the science of the human mind; but Hume far excelled him in the thoroughness and consistency with which he followed this way. Locke’s express purpose was to examine the understanding, that he might discover “the utmost extent of its tether.” He does not doubt that knowledge can signify a reality outside the mind; but he wishes to determine the range of this cognitive power. From the outset, Hume conceives the problem in a wider manner. All knowledge is a fact or process of human nature; if we are able, therefore, “to explain the principles of human nature,” we shall “in effect propose a complete system of the sciences.” Without doubt, this utterance points back to his early discovery of a “new medium by which truth might be established”—a discovery which, at the age of eighteen, had transported him beyond measure. In saying that “a complete system of the sciences” would result from “the principles of human nature,” Hume did not mean that the law of gravitation or the circulation of the blood could be discovered from an examination of the understanding and the emotions. His meaning was that, when the sciences are brought into system, certain general features are found to characterise them; and the explanation of these general features is to be sought in human nature—in other words, in our way of knowing and feeling. His statement, accordingly, comes simply to this, that mental science, or what we now call psychology, takes the place of philosophy—is itself philosophy.
Hume is commonly, and correctly, regarded as having worked out to the end the line of thought started by Locke. But, in the width of his purpose, the thoroughness of its elaboration and his clear consciousness of his task, he may be compared with Hobbes—a writer who had little direct effect upon his thought. For Hume is Hobbes inverted. The latter interprets the inner world—the world of life and thought—by means of the external or material world, whose impact gives rise to the motions which we call perception and volition. Hume, on the other hand, will assume nothing about external reality, but interprets it by means of the impressions or ideas of which we are all immediately conscious. And, as Hobbes saw all things under the rule of mechanical law, so Hume, also, has a universal principle of connection.
In enumerating these contents, he modifies the doctrine of Locke. According to Locke, the material of knowledge comes from two different sources—sensation and reflection. The view hardly admitted of statement without postulating both a mental and a material world existing over against one another. Hume tries to avoid any such postulate. His primary data are all of one kind; he calls them “impressions,” and says that they arise “from unknown causes.” Ideas are distinguished from impressions by their lesser degree of “force and liveliness.” Hume makes the generalisation that “every simple idea has a simple impression which resembles it”; an idea is thus the “faint image” of an impression; and there are degrees of this faintness: the “more lively and strong” are ideas of memory, the weaker are ideas of imagination. Further, certain ideas, in some unexplained way, reappear with the force and liveliness of impressions, or, as Hume puts it, “produce the new impressions” which he calls “impressions of reflection” and which he enumerates as passions, desires and emotions. Reflection is, thus, derived from sensation, although its impressions in their turn give rise to new ideas. All mental contents (in Hume’s language, all “perceptions”) are derived from sense impressions, and these arise from unknown causes. Simple ideas are distinguished from simple impressions merely by their comparative lack of force and liveliness; but these fainter data tend to group themselves in an order quite different from that of their corresponding impressions. By this “association of ideas” are formed the complex ideas of relations, modes and substances.
Such are the elements of Hume’s account of human nature; out of these elements, he has to explain knowledge and morality; and this explanation is, at the same time, to be “a complete system of the sciences.” He is fully alive to the problem. In knowledge, ideas are connected together by other relations than the “association” which rules imagination; and he proceeds at once to an enquiry into “all those qualities which make objects admit of comparison.” These, he calls “philosophical relations,” and he arranges them under seven general heads: resemblance, identity, space and time, quantity, degree of quality, contrariety, cause and effect.
All scientific propositions are regarded as expressing one or other of these relations. Hume regards the classification as exhaustive; and, at least, it is sufficient to form a comprehensive test of his theory. Since we have nothing to go upon but ideas and the impressions from which ideas originate, how are we to explain knowledge of these relations? Hume’s enquiry did not answer this question even to his own satisfaction; but it set a problem which has had to be faced by every subsequent thinker, and it has led many to adopt the sceptical conclusion to which the author himself was inclined.