Define "rationalist" as one who thinks that the truth of the matter is best arrived at through the use of reason rather than by empirical means.
For example, Plato says that by the mind alone is truth seen (Republic 527e) and that when one tries to distinguish, say, what is big from what is little, "the intellect [is] compelled to see big and little, too, not mixed up together but distinguished, doing the opposite of what sight did" (524c).
Descartes, Spinoza, and Leibniz stand in this tradition, too. (Probably many others as well: Aristotle, Aquinas, Husserl.)
So does Frank Sinatra. Proof text: "Why not use your mentality, step up, wake up to reality?"
Actually, Cole Porter is the rationalist since he wrote the lyrics. But why not suppose that Frank's cover of the song entails endorsement of its epistemology?