Weber’s compass is the emblematic instrument of measurement in the field of scientific psychology that emerged during the 19th century. After presenting the vital early work of Weber and the research conducted by his successors, we shall show that Binet was extremely interested in the issues associated with esthesiometry. His experimental research was to show that when we measure an individual’s tactile sensitivity, we are in fact performing an exercise in psychology since the decision “one point” or “two points” is in essence a problem of interpretation. He was to emphasize the existence of individual differences which are due, in particular, to the different approaches and interpretations of subjects when they are asked to perform a task requiring them to distinguish between two points applied to certain parts of the body. Thus it is evident that the role of cognition (interpretation, imagination, and selective judgement) is considerable in esthesiometry. Through his highly detailed analysis of esthesiometry, Binet thus anticipated the modern methods of tactile discrimination based on Signal Detection Theory (SDT) which make it possible to distinguish, in the responses, between what is due to the ability to transcode the sensory information and what is due to the processes responsible for interpreting this information.