Cf. Inferno, XXXIII, 46:
ed io sentii chiavar l'uscio di sotto
My external sensations are no less private to myself than are my thoughts or my feelings. In either case my experience falls within my own circle, a circle closed on the outside; and, with all its elements alike, every sphere is opaque to the others which surround it. . . . In brief, regarded as an existence which appears in a soul, the whole world for each is peculiar and private to that soul.
Eliot is referring us to
Dante Alighieri's poem
Inferno, Canto XXXIII,
The line quoted by Eliot can be translated as:
and I heard, at its outlet underneath lock'd up the' horrible tower
F[rancis] H[erbert] Bradley (1846-1924) was the British philosopher who wrote Appearance and Reality (1893).
See the Bio note on F.H. Bradley.