"There's not an atom of doubt about what I'm going to tell you," he began. "Not an atom. Unless I'm mistaken you saw for yourself this afternoon--though of course you didn't know what you were seeing. You simply thought I looked younger, didn't you?" I waited in silence. "And I fancy my manner got a bit on your nerves--does a bit now for that matter?" This also I let pass without remark. "Well, let's start from that point. You said I looked thirty-five. Well, it's just that that's getting on your nerves--the less amiable side of my character when I was thirty-five, and--and--well, when you go you might take that bottle of whisky with you and make me sign the pledge or something. I'm trying--I'm honestly trying--to hang on, you see." I sighed. "I wish you could make it a bit plainer," I said. "I'm making it as plain as I can. Is _this_ plain--that something's happened to me, I don't know what, and _I'm getting younger instead of older?_" "Derry----" I began, half rising; but he held up one heroically-moulded hand. "Let me finish. And if I happen to go to sleep suddenly you just walk straight out, do you hear? Walk right out and shut the door. You're to promise that. There are some things I won't ask even a pal to go through.... So there it is. Instead of getting older like everybody else I'm simply getting younger. I'm perfectly sober--I haven't had a drink for five days--and I tell you I shall go on till I'm thirty, and then twenty-five, and then twenty, and then, at sixteen or thereabouts--that fellow wasn't very sound on his ages to-night--I shall die. _Now_ have you got it?" Even about human nature there are some things that you have to accept as it were mathematically. I am no mathematician, but I do know (for example) that the common phrase "mathematically certain" is a misnomer. The whole essence of mathematics lies, not in its certainties, but in its assumptions, its power to embrace any concept whatever and pin it down in the form of a symbol. Once you have adopted the symbol you don't trouble about what lies behind it. You merely proceed to reason on it. It can only have been in some such way that I accepted Derwent Rose's mad statement and was willing to see what superstructure he was prepared to raise upon it. I was even able to speak in an almost calm and ordinary voice. "Tell me how you know all this," I said. He was logical and prompt.