 Millikan (2000) gives an extensive treatment of   concepts.[27]  In brief, her view is that conceptions play no role in determining the extensions of the concepts with which they are associated. Millikan’s theory presupposes innate learning mechanisms that are tuned to identify substances of different sorts in accord with certain principles. The relevant sort of substance is that which accounts for the past selective success of the learning mechanisms. For instance, some mental mechanisms might have been selected for recognizing faces of individuals in accord with certain principles of operation, and others might have been selected for recognizing animals of different species in accord with other principles of operation. These mechanisms can acquire the “purpose” to recognize something more specific, such as a particular individual’s face or animals of a particular species: this is because the mechanisms were selected for recognizing things in that domain (faces or animals) in accord with certain principles of operation and, in accord with those principles, they now have the have the “purpose” to recognize a particular individual’s face or animals of a particular species. The extension of a substance concept, Millikan tells us, is what substance it was selected to recognize.