But if we leave out of account this abiding continuity and self-identity of the subsisting individual substance or nature, which is the subject, cause and agent of these acts of memory and consciousness, how can these latter, in and by themselves, possibly form, or even indeed reveal to us, our personal identity? Locke felt this difficulty; and he tried in vain to meet it: in vain, for it is insuperable. He merely suggests that “the same consciousness ... can be transferred from one thinking substance to another,” in which case “it will be possible that two thinking substances may make [successively] one person”.(317) This is practically his last word on the question,—and it is worthy of note, for it virtually _substantializes consciousness_. It makes consciousness, which is really only an act or a series of acts, a _something substantial and subsisting_. We have seen already how modern phenomenists, once they reject the notion of substance as invalid or superfluous, must by that very fact equivalently _substantialize accidents_ (61); for substance, being a necessary category of human thought as exercised on reality, cannot really be dispensed with. And we see in the present context an illustration of this fact. The abiding self-identity of the human person cannot be explained otherwise than by the abiding self-identical subsistence of the individual human substance. If personal identity were constituted and determined by consciousness, by the series of conscious states connected and unified by memory, then it would appear that the human being in infancy, in sleep, in unconsciousness, or in a state of insanity, is not a human person! Philosophers who have not the hardihood to deny human personality to the individual of the human species in these states, and who on the other hand will not recognize the possession of a _rational nature_ or substance by the subsisting individual as the ground of the latter’s personality and personal identity, have recourse to the hypothesis of a _sub-conscious_, or “_sub-liminal_” _consciousness_ in the individual, as a substitute. If by this they merely meant an abiding _substantial_ rational principle of all mental activities, even of those which may be semi-conscious or sub-conscious, they would be merely calling by another name what we call the _rational nature_ of man. And the fact that they refer to this principle as the sub-conscious “self” or “Ego” shows how insistent is the rational need for rooting personality and personal identity in something which is a _substance_. But they do not and will not conceive it as a substance; whereas if it is not this, if it is only a “process,” or a “function,” or a “series” or “stream” of processes or functions, it can no more constitute or explain, or even reveal, personal identity, than a series or stream of _conscious_ states can.(318)