Our wake-a-day personality has no experience of it.

Then--and here the argument is irresistible--it must be another and distinct personality that falls when we are asleep, and that has had experience of such falling--that has, in short, a memory of past-day race experiences, just as our wake-a-day personality has a memory of our wake-a-day experiences.
It was at this stage in my reasoning that I began to see the light.

And quickly the light burst upon me with dazzling brightness, illuminating and explaining all that had been weird and uncanny and unnaturally impossible in my dream experiences.

In my sleep it was not my wake-a-day personality that took charge of me; it was another and distinct personality, possessing a new and totally different fund of experiences, and, to the point of my dreaming, possessing memories of those totally different experiences.
What was this personality?
When had it itself lived a wake-a-day life on this planet in order to collect this fund of strange experiences?
These were questions that my dreams themselves answered.

He lived in the long ago, when the world was young, in that period that we call the Mid-Pleistocene.