The failure of empiricism to account for mathematical ideas is due to its failure to connect them with acts performed. In accord with its sensationalistic character, traditional em- piricism sought their origin in sensory impressions, or at most in supposed abstraction from properties antecedently character- izing physical things. Experimental empiricism, has none of the difficulties of Hume and Mill in explaining the origin of math- ematical truths. It recognizes that experience, the actual expe- rience of men, is one of doing acts, performing operations, cutting, marking off, dividing up, extending, piecing together, joining, assembling and mixing, hoarding and dealing outj in general, selecting and adjusting things as means for reaching consequences. Only the peculiar hypnotic effect exercised by exclusive preoccupation with knowledge could have led think- ers to identify experience with reception of sensations, when five minutes' observation of achild would have disclosed that sensations count only as stimuli and registers of motor activity expended in doing things.