Experience has shown that no time in life precludes the acquirement of new knowledge and new habits by one who thinks it worth while to make the attempt. The elderly person will be surprised at his progress if he will bring to bear upon a new subject a mind free from doubts of its usefulness, doubts of his own ability, worry lest he is wasting valuable time, regrets for the past and plans for the future. It is not always possible to say just where useful habit merges into obsession. A certain individual, we will say, invariably puts on the left shoe before the right. This is a useful habit, fixed by constant repetition, useful because it relieves the brain of conscious effort. But suppose he decides some morning to put on the right shoe before the left; this new order so offends his sense of the fitness of things that he finds it hard to proceed; if he perseveres, his feet feel wrong to him; the discomfort grows until finally he is impelled to remove the shoes and replace them in the usual order. In this case an act which started as a useful habit has been replaced by an obsession. Suppose, again, a person obsessed by the fear of poison is prevented from washing his hands before eating. He sits down, perhaps, fully intending to proceed as if nothing had happened, but the thought occurs to him that he may have touched something poisonous, though his reason tells him this is most improbable. He reviews the events of the day and can find no suggestion of poison; still the thought of poison obtrudes itself, and he finds it impossible to put anything which he touches into his mouth. He next wonders if he has not already put something into his mouth. This thought produces a mental panic, the blood mounts to his head, he becomes incapable of coherent thought or speech, and the task of finishing his dinner would now be beyond his power even if he had not lost all taste for it. Such illustrations of obsession in daily life, by no means rare, could be multiplied indefinitely, and may be perhaps better appreciated than the text-book illustration of the man who neglected to flick off with his whip a certain stone from the top of a wall, and who could not sleep until he had returned to the spot and performed the act.