In this chapter Gellner describes and contrasts the views of Descartes, Hume and Kant in relation to their roles in practicing and promoting reason against culture (tradition, authority).

This is Gellner's account of Descartes as the champion of reason against culture. This interpretation of Descartes is well documented by quotations from Descartes' Meditations, but I still find myself with some reservations. I can't disagree that Descarte's is explicit in preferring to use reason in discovering the truth for himself rather than relying on what I tend to think of as "conventional wisdom" and he mentions as "custom". I'm not sure that I want this to be considered the same thing as culture, which surely includes both the prevailing views of any period and the revolutionary ones as well?

In this section Gellner introduces Hume into the discussion, and argues that though usually thought of as an empiricist opposed to Descartes rationalism, he was in fact aligned with Descartes in favour of reason against culture. The two "sides" here are for me rather too loosely characterised. Rationalists and empiricist can also be thought of as united against romanticism, this opposition again being not so easy to characterised clearly.

Gellner here identifies as the primary difference between Descartes and Hume, in effect, the latter's scepticism. I think the charter referred to in the title of this section must be some legitimising basis for the rational knowledge sought by Descartes, which Hume repudiates.

Kant's copernican revolution, was re-christened by Russell a counter-revolution, and Gellner here follows his lead. Its relevance here is to resetablish legitimacy for knowledge, by turning away from perception as the source of knowledge to a source "in the structure of the human mind".

The chapter is about whether and how we can obtain knowledge by rational means. On a superficial analysis Descartes thought he had done it, Hume denied the possibility, which was then re-instated by Kant, in a manner rather different from that of Descartes. Despite these apparant disagreements all three of these philosophers are for Gellner on the same side, debating the details but all engaged in the search for truth through reason.