As already remarked, Locke had not been wholly consistent with the standpoint of empiricism. Though conceding to material objects a decided superiority above the thinking subject, there was yet one point, viz., the recognition of substance, where he claimed for the thinking a power above the objective world. Among all the complex ideas which are formed by the subjective thinking, the idea of substance is, according to Locke, the only one which has objective reality; all the rest being purely subjective, with nothing actually corresponding to them in the objective world. But in the very fact that the subjective thinking places the conception of substance, which it has formed, in the objective world, it affirms an objective relation of things, an objective connection of them among each other, and an existing rationality. The reason of the subject in this respect stands in a certain degree above the objective world, for the relation of substance is not derived immediately from the world of sense, and is no product of sensation nor of perception through the sense. On a pure empirical standpoint—and such was Locke’s—it was therefore illogical to allow the conception of substance to remain possessed of objective being. If the understanding is essentially a bare and empty space, a white unwritten paper, if its whole content of objective knowledge consists in the impressions made upon it by material things, then must the conception of substance also be explained as a mere subjective notion, a union of ideas joined together at the mind’s pleasure, and the subject itself, thus fully deprived of every thing to which it could lay claim, must become wholly subordinated to the material world. This stride to a logical empiricism Hume has made in his criticism on the conception of causality.

David Hume was born at Edinburgh 1711. Devoted in youth to the study of law, then for some time a merchant, he afterwards gave his attention exclusively to philosophy and history. His first literary attempt was hardly noticed. A more favorable reception was, however, given to his “Essays,”—of which he published different collections from 1742 to 1757, making in all five volumes. In these Hume has treated philosophical themes as a thoughtful and cultivated man of the world, but without any strict systematic connection. In 1752 he was elected to the care of a public library in Edinburgh, and began in this same year his famous history of England. Afterwards he became secretary of legation at Paris, where he became acquainted with Rousseau. In 1767 he became under secretary of state, an office, however, which he filled for only a brief period. His last years were spent in Edinburgh, in a quiet and contented seclusion. He died 1776.

The centre of Hume’s philosophizing is his criticism of the conception of cause. Locke had already expressed the thought that we attain the conception of substance only by the habit of always seeing certain modes together. Hume takes up this thought with earnestness. Whence do we know, he asks, that two things stand to each other in the relation of cause and effect? We do not know it apriori, for since the effect is something other than the cause, while knowledge apriori embraces only that which is identical, the effect cannot thus be discovered in the cause; neither do we know it through experience, for experience reveals to us only the succession in time of two facts. All our conclusions from experience, therefore, rest simply upon habit. Because we are in the habit of seeing that one thing is followed in time by another, do we form the notion that the latter must follow out of the former: we make the relation of causality out of the relation of succession; but a connection in time is naturally something other than a causal connection. Hence, with the conception of causality, we transcend that which is given in perception and form for ourselves, notions to which we are properly not entitled.—That which belongs to causality belongs to every necessary relation. We find within us conceptions, as those of power and expression, and in general that of necessary connection; but let us note how we attain these: not through sensation, for though external objects seem to us to have coetaneousness of being, they show as no necessary connection. Do they then come through reflection? True, it seems as if we might get the idea of power by seeing that the organs of our body move in consequence of the dictate of our mind. But since we do not know the means through which the mind works, and since all the organs of the body cannot be moved by the will, it follows, that we are indeed pointed to experience in reference to this activity; but since experience can show us only a frequent conjunction, but no real connection, it follows also that we come to the conception of power as of every necessary connection, only because we are accustomed to a transcending process in our notions. All conceptions which express a relation of necessity, all knowledge presumptive of a real objective connection of things, rests therefore ultimately only upon the association of ideas. Having denied the conception of substance, Hume was led also to deny that of the Ego or self. If the Ego or self really exists, it must be a substance possessing inherent qualities. But since our conception of substance is purely subjective, without objective reality, it follows that there is no correspondent reality to our conception of the self or the Ego. The self or the Ego is, in fact, nothing other than a compound of many notions following rapidly upon each other; and under this compound we lay a conceived substratum, which we call soul, self, Ego (I). The self, or the Ego, rests wholly on an illusion. Of course, with such premises, nothing can be said of the immortality of the soul. If the soul is only the compound of our notions, it necessarily ceases with the notions—that which is compounded of the movements of the body dies with those movements.

There needs no further proof, than simply to utter these chief thoughts of Hume, to show that his scepticism is only a logical carrying out of Locke’s empiricism. Every determination of universality and necessity must fall away, if we derive our knowledge only from perceptions through the sense; these determinations cannot be comprised in sensation.