Esoteric is a fuzzy concept to define, there are no clear cut boundaries. I think that's right.
So we have something like a "heaps" (sorites) situation. How many grains of sand, for instance, does it take to make up a heap?
I met a schizophrenic poet once, and I was indirectly alluding to some mystic experience i had, via poetry, and he said "Hey I know what you mean about the light". I have a mystical conversion in a storm, and he had similar looking at light through a window pane.
So what is the explanation for this "esoteric knowledge". Schizophrenia? That depends on cultural setting and standards, and the openess to mysticism in the institutions of psychiatry.
But if the consensus were it is menatal illness, suddenly we have esoteric (private hidden etc.) points of reference which - ok, lets hear it, from a secular perspective - are suddenly the gambit of the scientist.