Full Idea
In Hume's account of action, practical reason is not a very forceful guide to conduct, since we can escape its demands by abandoning or modifying our desires.
Gist of Idea
For Hume, practical reason has little force, because we can always modify our desires
Source
report of David Hume (Treatise of Human Nature [1739]) by Gordon Graham - Eight Theories of Ethics Ch.6
Book Reference
Graham,Gordon: 'Eight Theories of Ethics' [Routledge 2004], p.107
A Reaction
Presumably a desire can be a good reason, and we can passionately desire to be rational, etc., so this is a rather complex issue. 'Pure reason' is not 'all-or-nothing', and neither is pure desire.