What, then, is the barrier between the animals and man, that excludes the animals from man’s mental world? It is the barrier primarily of Space and Time. This, I think, can be made clear.

First, consider Time. It is clear, to begin with, that the dog holds in his memory, at least in some dim way, the time-span, or the time experiences, within the physical duration of his own life. The dog will go to-day where he found food yesterday. Darwin’s dog recognized him after an absence of five years and two days. Odysseus’s dog, Argus, according to Homer, recognized his master, even through his beggar’s disguise, after an absence of twenty years. There is no doubt that the dog has memory of this kind, which shows that he has some grasp of time within his own life-span. Whether it is conscious memory with an explicit measurement of a stretch of time between a point in past time and the present moment, as is the case with man’s memory, is extremely doubtful. It seems to be rather a recognition of the master’s identity, or sameness, through a succession of experiences, each of which is for the dog a present experience without any explicit differentiation of time into past and present.