The analogy may be carried further. As it is the average of the physical food you digest which ultimately determines the constitution of your body, so it is the average of the mental food you absorb which determines the constitution of your mind. One good meal will not offset a week of bad ones; one good book will never offset any number of poor books. Further, as no one has a perfect memory, you do not retain all you read any more than you retain all you eat. Therefore if you do not want your mind to retrogress, you should not rest satisfied with books already read, but should continue to read books at least as good as any previous. As at any given time your bodily health—so far as it depends on food—is mainly determined by the meals of the last few days or weeks, so is your mental health dependent on the last few books you have read. One of the first things we should look to in selecting books is their comprehensiveness. To quote Arnold Bennett: “Unless and until a man has formed a scheme of knowledge, be it but a mere skeleton, his reading must necessarily be unphilosophical. He must have attained to some notion of the interrelations of the various branches of knowledge before he can properly comprehend the branch of knowledge in which he specializes.”[23] As an aid in forming this scheme of knowledge, Mr. Bennett suggests Herbert Spencer’s _First Principles_. I heartily endorse his choice. I would add to it the essay on _The Classification of the Sciences_ by the same author. These works are classics, and one of the most regrettable of difficulties is that of getting people to read the classics. Mention to a man Darwin’s _Origin of Species_ or _Descent of Man_, and he will reply, “Oh, yes, that’s the theory that says men descended from monkeys.” Satisfied that he knows all there is to know about it, he never reads any of Darwin’s works. Now passing over the fact that the theory does not assert that man descended from monkeys and never intended to assert it;—what a compliment to Darwin’s thought and brevity to assume that all his books can be summed up in a phrase! But Darwin is not the only sufferer. If we come across the title of a classic often enough, and hear a lot of talk “about it and about” and a few quotations from it, we gradually come to believe we know all the contents worth knowing. This is why Shakespeare, and in fact most of the classics, are so seldom actually read, and why we go for our serious reading to a book on “How to Read Character from Handwriting” or to a sensational volume on prostitution by one of our modern “sociologists.” The only way we can keep ourselves from such stuff is to lay out some definite end, some big objective, to be attained; and before reading a book we should ask how that helps us to attain it.