Modern critics have rightly insisted, as against Hume, that isolated perceptions without a self are abstractions not less unintelligible than a self without perceptions. But the metaphysical argument for human immortality has not benefited by this more concrete interpretation of epistemology; and probably Hume was really more interested in destroying this than in maintaining the sceptical paradox which does not recur in his later writings.
A word must be added about Hume's division of perceptions into impressions and ideas. The point left out of sight in this analysis is that impressions of sense habitually find their reflexes not in revived sensations, but in expressions, in motor reactions which, with human beings, mostly take the form of words uttered or thought. These, no doubt, are associated to some small extent with revived sensations; but they are more commonly grouped with other words, with movements of the limbs, and with actions on the material or human environment of the percipient. Such expressions are incomparably easier to revive in memory, imagination, or expectation than the impressions that originally excited them; and, indeed, it is in connection with them that such revivals of sensation
as we actually experience take place. And it is probable that to this active side of our consciousness that we may trace those associative processes which Hume studies next in his analysis of human knowledge.
Putting aside principles of doubtful or secondary value, the relations between states of consciousness that first offer themselves to view are, according to Hume, Co-existence and Succession (united under the name of Contiguity), Resemblance, and Causation. It is with the account he gives of this last category that his name is inseparably associated, for from it all subsequent speculation has taken rise. Yet primarily he seems to have had no other object in view than to simplify the laws of knowledge by resolving one of them into a particular case of another, and thus reducing his three categories to two. The relation of cause and effect, he tells us, is no more than a certain relation between antecedent and consequent in time where the sequence is so habitual as to establish in our minds a custom of expecting the one whenever the other occurs. The sequence is not necessary, for one can think, without any self-contradiction, of a change which has not been preceded by another change; nor is it, like the truths of geometry, something that can be known à priori. Without experience no one could tell that bread will nourish a man and not nourish a lion, nor even predict how a billiard-ball will behave when another ball strikes it. Should it be objected that the à priori knowledge of a general principle need not involve an equal knowledge of nature's operations in particular cases, Hume would doubtless reply by saying that there is no abstract idea of causation apart from its concrete exemplifications.
It is possible to accept Hume's theory in principle
without pledging oneself to all his incidental contentions. Causation, as a general law, may be known only by experience, whether we can or cannot think of it as a pure abstraction. And we may interpret it in terms of unconditional antecedence and consequence, while discarding his apparent assumption of an inscrutable connection between the two; a mysterious necessity for the production of the one by the other, for which it is felt that a reason exists, but for which our reason cannot account. It is inconceivable that our knowledge of any given sequence could be increased, except by the disclosure of intermediate sequences, making their continuity, in space and time, more absolute than we had before perceived, until the whole process has been resolved into a transference of momentum from one molecule to another—a change for which, according to Hume, no reason can be given. Nor, on his principles, would it help us to explain such transferences by bringing them under the law of the Conservation of Energy. For, although this would be a great triumph for science, his philosophy demands a reason why the quantity of energy should remain unalterable for ever.
It is a mistake, shared by Hume with his opponents, to suppose that the common sense of mankind ever saw more than invariable sequence in the relation of cause and effect, or ever interpolated a mysterious power between them. In the famous verse, "Let there be light, and there was light," it is the instantaneity of succession, not the interpolation of any exerted effort, that so impresses the imagination. And when Shakespeare wants to illustrate logical compulsion in conduct, his reference is to an instance of invariable succession:—
This above all,—to thine own self be true;
And it must follow, as the night the day,