The inquiry that follows is a philosophical homecoming. At any rate, it is an attempt to return to the place whence I set out in my teens, dozens of published books, and many more dozens of unpublished manuscripts, ago.

My philosophical awakening was shaped by fears. Some were well-founded, such as that sooner or later I, and everyone I cared for, would die in pain and terror. Other fears were equally commonplace but less well-founded: that my freedom was illusory; and that the world as I experienced it might be a dream. These philosophical concerns were, a few years later, offset by something quite different: an overwhelming astonishment that I existed, that I could think that I existed; that I could make everyday sense of myself and my world; in short, that there is “that”. It was accompanied by an unexpected joy that I still cannot fully explain although I am grateful for it.

One moment I have recalled so frequently that not one detail will have survived undistorted. As I remember it now, I am standing in a little attic room, fragrant with cabbage soup and the fishy smell of a malfunctioning anglepoise lamp. These were my student digs: 60 Park Town Crescent, Oxford, where I was lodged with Mrs Pasternak-Slater, the exiled sister of Boris. I am talking to my friend Chris Verity, then a medical student and now a retired consultant. I can't remember what we were talking about. Suddenly I heard my own voice and, beyond my voice, the “that” implicit in it – located somewhere between “that I am”, “that x is the case”, “that the world exists” and “that I am making sense of something”. Thus, my encounter with “thatter”. “Mystery” is, perhaps, too narrow a word; rather a revelation tinged with surprise, feeling itself to be on the threshold of a further revelation that in the subsequent fifty or more years has not revealed itself.