This conclusion successively considers four points: it first attempts to analyse the main shortcomings of the study while remaining within the justificatory framework of Chapter 1 (A); it then provides a critical assessment of the polar assumptions of chapter 1 (B); third, it shows that our 'monist' option stressing theoretical unity has led us to interpret the results in a much too restrictive way (C); finally it discusses some of the most promising directions of research in the theory of second-best taxation (D).

(A) The model under consideration in chapters 2,3 and 5 has features which fit the conclusions of chapter 1 except in one respect: there are a number of commodities for which taxation should be non-linear rather than linear. Whatever the merits of the linearity assumption, non-linear taxation of, for example, labour income is suggested both by theoretical analysis and real world observation. Even taking into account the detailed analysis of quotas policies - that introduce non-linearities in the tax system, albeit of a special kind - in the section 4.2 of chapter 4, and the discussion of the non-linear taxation model of section 4.4, the treatment of non-linearities in this book is incomplete. This is an obvious shortcoming.