"Now," he said. "You see that notch I've made. That's my present age--thirty-three--dead in the middle of the lighted portion. Now let's start. First of all I've got two memories. I've got one in each direction. I'm the only man who has. And this part of the edge that the torch lights up is my total range both ways. Now watch me move the torch. If I move it your way"--he did so--"I get more of memory 'A' ('A' for Age) and less of memory 'B' ('B' for Boyhood). And if I move it my way"--he moved it his way--"I get less of 'A' and more of 'B.' See?" I saw. I began to wish I didn't. "Very well," he went on. "Obviously it's for me to decide where I want to stop, and then--to do so if I can. And now the bother begins. If--that--scale--could be numbered properly"--he divided the words as I have divided them, and I felt cold at the intensity of his emphasis--"if it could be divided as I want it divided, with thirty-three dead in the middle--then forty-five would come _here_." He crossed his left hand over the one that held the torch, as a pianist picks out a single treble note, and dug another nick at my end of the illuminated portion. "Now," he continued, "let's see what the figure would be at my end. Forty-five less thirty-three is twelve, and twelve from thirty-three's twenty-one. It would be twenty-one." He registered another notch, this time at his own end. "But"--swiftly he slid the torch his way--"twenty-one's no good to me at all. No more good than a sick headache. I've got to be younger than that. You see what I've got to do. I've got to combine the two maximum phases of myself if I'm to write that book. But at the same time I've got to write it when I _did_ write that kind of thing before. What does that mean? Where's a bit of paper?" He set the torch down on the table, where it made a vivid flat parabola of light, and took an envelope from his pocket. In the semi-darkness he began to jot down figures. "Here you are. Just a few specimen numbers for trial and error. I'm assuming that the scale's capable of regular division, which it isn't, for many reasons; but let's take it in its simplest form. 16:33:50--21:33:45--30:33:36