This chapter views that the proper objects of economic theory are such selection processes instead of deliberation processes of agent's selectionism. Reinterpreting economic theory to be about the results of selection processes could be possible because both the selection theory and the economic theory of choice are theories of optimisation. Sidney Winter explores under which conditions selection mechanisms can ensure the applicability of standard economic theory by guaranteeing that only firms who act as profit maximisers will survive in the long term in the market. He comes to the result that selection considerations support the use of traditional economic theory only under special circumstances and for a restricted range of applications. Certain features of markets might either make these psychological influences on individual choice behaviour disappear; another is that agents who act according to standard economic theory have a stronger influence on market outcomes than agents who deviate from the behaviour predicted by the theory.