A chicken, for example, hides under some cover the first time it hears the cry of a hawk; it scratches the first time its feet touch sand or gravel; it pecks the first time it sees an insect near by. An infant closes its eyes the first time it feels cold wind blow upon them; it cries the first time it feels pain; it clasps its fingers together the first time a touch is felt inside them. The child's nervous system is so organized that, in each of the cases named, the stimulus brings forth the particular, definite response. These acts do not have to be learned. But it is quite different in typewriting and piano playing. One _must learn_ what keys on the piano to strike in response to the various situations of the notes as written in the music. One must also learn the keys on the typewriter before he can operate a typewriter. And in the case of other habits, we find, for example, that one does not respond by saying "81" for 9 times 9; nor "13" for 6 plus 7; nor "8" for 15 minus 7; nor "8" for the square root of 64; nor "144" for the square of 12, etc., until one has learned in each case. Some connections between stimulus and response we have through inheritance; all others are built up and established in one's lifetime, particularly in the first thirty years of one's life. We have spoken of bonds between stimulus and response, but have not explained just what can be meant by a _bond_. In what sense are stimulus and response bound together? A bond is a matter of greater permeability, of less resistance in one direction through the nervous system than in other directions. Nerves are conductors for nerve-currents. When a nerve-current is started in a sense organ, it passes on through the path of least resistance. Now, some nerves are so organized and connected through inheritance as to offer small resistance. This forms a ready-made connection between stimulus and response. Muscular responses that are connected with their stimuli through inherited bonds, by inherited nerve structure, are called instincts. Those that are connected by acquired bonds are called habits. Sucking, crying, laughing, are instinctive acts. Adding, typewriting, piano playing, are habits. The term _instinct_ may be given to the act depending upon inherited structure, an inherited bond, or it may be given to the inherited bond itself. Similarly, the term _habit_ may be given to an act that we have had to learn or to the bond which we ourselves establish between response and stimulus. In this book we shall usually mean by instinct an action depending upon inherited structure and by habit an act depending upon a bond established during lifetime. A good part of our early lives is spent in building up bonds between stimuli and responses. This establishing of bonds or connections is called _learning_.