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).