 The most interesting examples are collections of sensations each of which varies from all the others in the same respect, e.g., warmth, length, or duration. Sensations vary “in kind and degree”, as Shepherd puts it. All the sensations in the first collection just mentioned are alike in being warm, but differ in respect of degrees of warmth. It follows that their causes are alike in respect of some general property that admits of degrees and different in its degrees. The external gradable property cannot possibly be like a sensation, but it imparts an order among its grades which is like the order among their effects. As Shepherd says, the causes of sensations must have “the same proportions, in relation to each other among themselves, as the effects have to each other” (EASP: 28–9). So time, the continuously existent nature that has a part in causing sensations that arise successively, consists of positions with mutual relations proportional to the relations among the corresponding sensations. Time is, then, capable of being measured by the idea of a continued external object that undergoes changes of a sort deemed suitable to measure the changes in other externally existing things. The durations of the perceptions of, say, beats of a pendulum thus serve to measure the durations of the beats that cause them. The grades of difference between elements on the corresponding sides can be as small or large as observation and reason require. In addition to similarity, variety, and proportionality, and contrariety are mentioned as a relation common to sensations and their causes: the causes of sensations of motion and rest, solidity and lack of resistance to motion, extension and inextension, red and blue, different places, etc. must be related as contraries. 