A first or preliminary edition of this book, in mimeographed sheets, was in use for two years in introductory classes conducted by the author and his colleagues, and was subjected to exceedingly helpful criticism from both teachers and students. The revision of that earlier edition into the present form has been very much of a coöperative enterprise, and so many have coöperated that room could scarcely be found for all their names. Professor A. T. Poffenberger, Dr. Clara F. Chassell, Dr. Georgina I. Gates, Mr. Gardner Murphy, Mr. Harold E. Jones and Mr. Paul S. Achilles have given me the advantage of their class-room experience with the mimeographed book. Dr. Christine Ladd-Franklin has very carefully gone over with me the passages dealing with color vision and with reasoning. Miss Elizabeth T. Sullivan, Miss Anna B. Copeland, Miss Helen Harper and Dr. A. H. Martin have been of great assistance in the final stages of the work. Important suggestions have come also from several other universities, where the mimeographed book was inspected. R. S. W. Columbia University August, 1921 CONTENTS CHAPTER I PAGE WHAT PSYCHOLOGY IS AND DOES 1 Varieties of Psychology 2 Psychology as Related to Other Sciences 5 The Science of Consciousness 7 The Science of Behavior 8 Introspection 10 Objective Observation 11 General Lines of Psychological Investigation 14 Summary and Attempt at a Definition 17 Exercises 19 References 20 CHAPTER II REACTIONS 21 The Reaction Time Experiment 22 Reflex Action 24 The Nerves in Reflex Action 26 Internal Construction of the Nerves and Nerve Centers 31 The Synapse 34 Coördination 37 Reactions in General 39 Exercises 42 References 44 CHAPTER III REACTIONS OF DIFFERENT LEVELS 45 Different Sorts of Stimuli 47 The Motor Centers, Lower and Higher 49 How the Brain Produces Muscular Movements 53 Facilitation and Inhibition 54 Super-motor Centers in the Cortex 56 Speech Centers 57 The Auditory Centers 59 The Visual Centers 62 Cortical Centers for the Other Senses 68 Lower Sensory Centers 64 The Cerebellum 66 Different Levels of Reaction 65 Exercises 67 References 67 CHAPTER IV TENDENCIES TO REACTION 68 Purposive Behavior 70 Organic States that Influence Behavior 72