Written by one of today's most creative and innovative philosophers, Ruth Garrett Millikan, this book examines basic empirical concepts; how they are acquired, how they function, and how they have been misrepresented in the traditional philosophical literature. Millikan places cognitive psychology in an evolutionary context where human cognition is assumed to be an outgrowth of primitive forms of mentality, and assumed to have 'functions' in the biological sense. Of particular interest are her discussions of the nature of abilities as different from dispositions, her detailed analysis of the psychological act of reidentifying substances, and her critique of the language of thought for mental representation. In a radical departure from current philosophical and psychological theories of concepts, this book provides the first in-depth discussion on the psychological act of reidentification.