Different theories of cognitive science propose different system descriptions in their models for the explanation of cognitive phenomena. According to one view, they are incompatible and competing theories. The view is defended by theorists and philosophers from different perspectives and they all claim that the proper conception of cognition is the conception provided by the theory which they advocate. The other view, on the other hand, insists on the compatibility of those theories. According to this view which is also defended here, these different theories are not only compatible, but also they are complementary. The cooperation of these theories and integration of the conceptions provided by these theories are needed to have a full account of cognition.