except we suppose it to be causal--which would be a reassertion of the law--he does not explain; nor yet what warrant we have for assuming that causation holds through all time, or at any future moment of time. Next to the law of universal causation inductive science rests on the doctrine of natural kinds. The material universe is known to consist of a number of substances--namely, the chemical elements and their combinations, so constituted that a certain set of characteristic properties are invariably associated with an indefinite number of other properties. Thus, if in a strange country a certain mineral answers the usual {135} tests for arsenic, we know that a given dose of it will destroy life; and we are equally certain that if the spectroscopic examination of a new star shows the characteristic lines of iron, a metal possessing all the properties of iron as we find it in our mines is present in that distant luminary. According to Mill, we are justified in drawing that sweeping inference on the strength of a single well-authenticated observation, because we know by innumerable observations on terrestrial substances that natural kinds possessing such index qualities do exist, whereas there is not a single instance of a substance possessing those qualities without the rest. For Mill, as for Hume, reality means states of consciousness and the relations between them. Matter he defines as a permanent possibility of sensation; mind as a permanent possibility of thought and feeling. But the latter definition is admittedly not satisfactory. For a stream of thoughts and feelings which is proved by memory to have the consciousness of itself seems to be something more than a mere stream. All explanations must end in an ultimate inexplicability. God may be conceived as a series of thoughts and feelings prolonged through eternity; and it is a logically defensible hypothesis that the order of nature was designed by such a being, although the amount of suffering endured by living creatures excludes the notion of a Creator at once beneficent and omnipotent. And if the Darwinian theory were established, the case for a designing intelligence would collapse. Personally Mill believed neither in a God nor in a future life.