It is now widely agreed that current ability measures reflect a complex interaction of environment with genetic potential. This leads to a basic measurement problem since persons with the same measured ability may vary widely in potential due to non-equivalent learning opportunities. The purpose of this paper is to present a model which may hold some promise in psychometrically distinguishing ability (current status) from aptitude (potential). Data on a simple ability are analyzed according to the model to illustrate how some of the practical problems may be solved. (Author)