Full Idea
Descartes rejects logos because it does not achieve the certainty he craves. He replaces it with his own model of rationality, one modelled essentially on mathematics.
Gist of Idea
Descartes tried to model reason on maths instead of 'logos'
Source
comment on René Descartes (Meditations [1641], §1) by David Roochnik - The Tragedy of Reason p.76
Book Reference
Roochnik,David: 'The Tragedy of Reason: the Platonic logos' [Routledge 1990], p.76