Full Idea
We must assume that something matters - that some things are for better and some things are for worse, for without that our assumed rationality would have nothing on which to get a purchase.
Gist of Idea
Rationality requires the assumption that things are either for better or worse
Source
Daniel C. Dennett (Elbow Room: varieties of free will [1984], §7.1)
Book Reference
Dennett,Daniel C.: 'Elbow Room - Free will worth wanting' [MIT 1999], p.155
A Reaction
It does seem that rationality wouldn't exist as an activity without some value to motivate it.