"Free will" may be. But "rationality" can be given a clear, concise definition that most of us will agree upon in our context. Here, amichail probably talks about epistemic rationality: the art of obtaining beliefs that correspond to reality as closely as possible.

Source: http://lesswrong.com/lw/31/what_do_we_mean_by_rationality/

Now, the question of free will itself has been discussed. here: http://wiki.lesswrong.com/wiki/Free_will and there: http://wiki.lesswrong.com/wiki/Free_will_%28solution%29 are probably very good start points.

I haven't read these pages yet, but from what I have already read on http://lesswrong.com (which is quite a lot) is of outstanding quality.