When some thought has occurred to us we believe at the time we are thinking it that it is ours forever. We cannot conceive that it shall ever be forgotten. Perish that belief! I have sometimes had an idea occur to me (really!), and have believed it absolutely new, at least so far as I was concerned. But on looking over things written before, I have found that I had had almost identically the same thought at another time. Not only did I forget the idea; I did not even recognize it at its second appearance. To be sure, in these cases the thoughts came a second time. But thoughts are seldom so obliging. Therefore when an idea occurs or when you have solved a problem, even a problem suggested by a book, you should immediately put the idea or solution in writing. You may of course wait until the end of the day. But the safest way of capturing an idea is to write it the minute after it flashes through your brain, or it may be lost forever. It was with this in mind that in the chapter on reading I advised immediately writing not only ideas but problems which occurred to one. The discovery of a new problem is just as important and necessary for intellectual advance as the solution of an old one. If we do not write our problems we are apt to forget they exist; we put ourselves in danger of assuming without question some proposition which is not true. To facilitate the writing of your thoughts and meditations I suggest a notebook kept specially for that purpose. In addition to this you should always carry about with you some blank paper and a pencil, so as to be ever ready to jot down anything. To write an idea does not of course imply that you cannot later reject it, or change it, or develop it further. The elusiveness of thoughts is most strikingly brought out when writing them down. When we are writing a long sentence we have in mind the exact words with which we are going to finish it. But our attention is called for the moment to the physical act of writing, and presto!—the words are gone; we are compelled to end our sentence in a different way. I have mentioned the advantages of shorthand and typewriting for keeping pace with thought. I need merely repeat my advice to use these acquirements if you have them. Thoughts, I must repeat, are fleeting. No device for trapping them should be despised. Not least among the advantages of a notebook in which to write thoughts is the permanent historical record it gives. Every thought we write should be dated, day, month and year, like a letter. When we come to read over ideas jotted down from time to time in this manner, we shall see before us an intellectual autobiography. We shall see how our recent thoughts compare with those written sometime ago. We shall see just what our opinions were at certain times, and how they have changed. And we shall see whether our mental progress has been marked, or whether we have been standing still.