In a given sense-history there is one and only one time-direction. This is because the simultaneity or successiveness of sensa in the same sense-history is actually sensed, and we have therefore no choice as to which we shall group together as simultaneous, and which we shall group apart as successive. The succes­ sive slabs of the sense-history are given to us in the form of sense-fields, and the only possible time-direction is that of their common normal. The only choice allowed to us is that we could take any straight line in the sensehistory, parallel to the time-direction, as a permissible time-axis, assuming that the geo-chronometry of the sense-history is Euclidean.