The Collaborative International Dictionary of English v.0.48:

Rationalistic \Ra`tion*al*is"tic\
   (r[a^]sh"[u^]n*al*[i^]s"t[i^]k), Rationalistical
\Ra`tion*al*is"tic*al\ (-t[i^]*kal), a.
   Belonging to, or in accordance with, the principles of
   rationalism. -- Ra`tion*al*is"tic*al*ly, adv.
   [1913 Webster]