There will be marked individual differences in the material selected, in the length of time any type of material will be utilized, and in the early constructive habits which will arise with respect to all materials worked with by the infant. Without instruction one infant (eighteen to twenty months in an observed case) will build a neat wall with her blocks, with one color always facing her. If the block is turned while she is not looking she will quickly change it and correct the defect. In other children such a bit of behavior can be inculcated only with the greatest difficulty. Still another child cannot be made to play with blocks but will work with twigs and sticks by the hour. Variations in the selection and use of material are the rule in infancy, but until we have followed up the future course of such variations upon _infants whose past we have watched day by day_ we are in no position to make generalizations about the original tendencies which underlie the vocations. (3) Finally, until we have obtained data upon the emotional life of the infant and the normal curve of instinctive and habit activity at the various ages, new methods for correcting deviations in emotional, instinctive and habit development cannot be worked out. Let us take a concrete example. A certain child is afraid of animals of every type, furry objects, the dark, etc. These fears are not hereditary. Our experiments will be convincing upon that point. What steps can we take to remove these fears which, unless they are removed in infancy, may become an enduring part of the child’s personality?[113] It will be seen that the Watsons are here studying attitudes—what ones appear, in what order they appear, what ones are universal, and what ones are particular to certain children. They introduce values—the materials and influences—only as means of determining the presence of attitudes, of calling them into action, of modifying them, and of giving them application in different fields. They consciously introduce change on the basis of some hypothesis and measure the effect of the new influences on personality development. Already in this brief passage and this unfinished experiment they indicate methods of determining (1) occupational aptitudes, (2) defective mental efficiency, (3) the age levels at which influences are to be presented and the order of their presentation, and (4) the conditions of stable or neurotic personalities. Their task is the measurement of influence under conditions which they prepare and control. All scientific experimentation involves the measurement of influence. The chemist measures the influence of a material, say coal tar; the technician uses this influence in preparing a dye or a medicine. The measurement of the influence is the definition of the situation preliminary to action.