Full Idea
By showing that subjective things are the most certain, Descartes performed a great service to philosophy.
Gist of Idea
Descartes showed that subjective things are the most certain
Source
Bertrand Russell (Problems of Philosophy [1912], Ch. 2)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.8
A Reaction
This praise comes from an empiricist, who has just said that 'sense-data' are the most certain things. I presume that animals are more certain of the world than they are of subjective things. In fact, probably on philosophers agree with Russell.