In this paper we will analyze some methodological and structural limits of mainstream economics. In the first part we will show that implicit philosophical and methodological assumptions affect its conceptual premises (naturalization of the capitalist society), and how the theory is not able to justify all its categories (in particular externality). In the second part we will see that, even if we accept its premises, the theory is not capable of demonstrating its theses, especially the concept of equilibrium; some of its deductions (including mathematical) are possible only assuming arbitrary ad hoc conditions. We conclude that the foundations of the mainstream economic theory and its capacity to interpret reality are questionable.