[1] ‘Animal Intelligence: An Experimental Study of the Associative Processes in Animals’ (’98), ‘The Instinctive Reactions of Young Chicks’ (’99), ‘A Note on the Psychology of Fishes’ (’99), and ‘The Mental Life of the Monkeys’ (’01). I have added a theoretical paper, ‘The Evolution of the Human Intellect,’ which appeared in the _Popular Science Monthly_ in 1901, and which was a direct outgrowth of the experimental work. I am indebted to the management of the _Psychological Review_, and that of the _American Naturalist_ and _Popular Science Monthly_, for permission to reprint the three shorter papers. [2] Unless one assumes telepathic influences. [3] Reason in Common Sense, p. 154 ff. [4] This chapter originally appeared as Monograph Supplement No. 8 of the Psychological Review. [5] I do not mean that scientists have been too credulous with regard to spiritualism, but am referring to the cases where ten or twenty scientists have been sent to observe some trick-performance by a spiritualistic ‘medium,’ and have all been absolutely confident that they understood the secret of its performance, _each of them giving a totally different explanation_. [6] The phrase ‘practically utter hunger’ has given rise to misunderstandings. I have been accused of experimenting with starving or half-starved animals, with animals brought to a state of fear and panic by hunger, and the like! The desideratum is, of course, to have the motive as nearly as possible of equal strength in each experiment with any one animal with any one act. That is, the animal should be as hungry at the tenth or twentieth trial as at the first. To attain this, the animal was given after each ‘success’ only a very small bit of food as a reward (say, for a young cat, one quarter of a cubic centimeter of fish or meat) and tested not too many times on any one day. ‘Utter hunger’ means that no diminution in his appetite was noted and that at the close of the experiment for the day he would still eat a hearty meal. After the experiments for the day were done, the cats received abundant food to maintain health, growth and spirits, but commonly somewhat less than they would of their own accord have taken. No one of the many visitors to the room mentioned anything extraordinary or distressful in the animals’ condition. There were no signs of fear or panic. Possibly I was wrong in choosing the term ‘utter hunger’ to denote the hunger of an animal in good, but not pampered, condition and without food for fourteen hours. It is not sure, however, that the term ‘utter hunger’ is inappropriate. The few reports made of experiments in going without food seem to show that, in health, the feeling of hunger reaches its maximum intensity very early. It is of course not at all the same thing as the complex of discomforts produced by long-continued insufficiency of food. Hunger is not at all a synonym for starvation.