Two economic approaches to human behaviour in liberalism -- The object of economic study -- The scope of economics -- The characterization of economic behaviour -- The economic approach of Mises : praxeology -- The definition of economic behaviour in the work of Mises -- The structure of the work of Mises -- Development of the subjective theory of value -- The Misian theoretical system -- The Misian methodology -- The structure of the general theory of action of Mises -- Economic act and economic action -- The axiom of action -- The action and the socio-cultural framework -- The praxeological categories : causality and time -- The discovery of means and the creation of possibilities -- Introduction -- Kirzner's pure entrepreneurship -- Development of the concept of pure entrepreneurship -- The scope of pure entrepreneurship in Kirzner's work -- The entrepreneurial function and the socio-cultural framework of individual action -- The market as a social institution -- The evaluative system -- The praxeological relation between ethics and the market -- Private property in praxeology -- Causality as a praxeological category -- Causality in the work of Mises -- The Misian basis of causality -- The Misian determination of the cause -- The project -- The constituents of the project -- The temporal structure of the project -- The structure of the information in the project -- The execution of the project -- The economic approach of Becker : the generalization of homo economicus -- The definition of economic behaviour in the work of Becker -- Introduction -- The extension of the theory of utility -- Becker's theoretical model -- Critical analysis of Becker's definition of economic behaviour : stability of preferences -- The relation between Becker and Friedman -- Critical analysis of the theoretical approach of Becker to human behaviour -- Critical analysis of Beckers definition of economic behaviour : maximizing behaviour and market equilibrium -- The Beckerian explanation of prices -- Critical analysis of the Beckerian theory of prices -- The Austrian explanation of prices -- Conclusions -- Theoretical differences -- Methodological differences.