Habit has been defined as second nature but it has also been called ten times nature. It consists in the automatic response to a stimulus; and its basis is the plasticity of the nervous system, which receives and records the stimulus. Associated with this is the corresponding action, and when this whole process of stimulus and action has been carried through in the same way a sufficient number of times, a brain-track, path, or channel is made, along which the thoughts run with ever-increasing ease and readiness. On the sea coasts the tree bends over to the prevailing wind and finally acquires a permanent curve in that direction, so the rearrangement of cells, and the modification of the structure of the brain, gradually gives the mind itself a bent. Another instance of such modification of structure is the crease that remains in a piece of paper after we have once folded it. In some such way as this, then, the mind is gradually moulded along the line of its habits, and it may truly be said of most of us that we are ninety per cent, habit. But we must, at all costs, endeavour to keep the remaining ten per cent, original, so that we do not become habit to the whole hundred per cent. To make no habits at all is impossible, and our main preoccupation must be to select 151those habits which are useful, and to avoid those which can harm us. Habit is a most useful servant but an impossible master.