A cognitive modeling approach coming from ergonomics is presented. It is based on an activity analysis that starts from a “trace of activity” made up of data recorded with sensors. Knowledge engineering software was developed to transform this trace and map it with explicative concepts. These concepts are defined by cognitive ergonomists to explain the activity by the mental states and processes of the operator. These concepts are produced on a pragmatic and evolutionist basis with the interactive help of the software, and are confronted with the operator’s own assessment of his subjectivity. The approach is illustrated by its application for car driver cognitive modeling.