People often speak as though acting naturally were the opposite of acting rationally.  For the other animals, it is.  For humans, it isn’t.  Because we have a rational nature, all of our natural inclinations must pertain to reason in some way, or they would not be natural to us.

But there are a few twists.  Here is the first twist:  We are not pure intelligences; for example, we also share in the nature common to all animals.  Here is the second one:  In us, the inclinations we share with other animals are taken up into reason and transformed by it.  Thus, we experience them differently than other animals do.  Both humans and other animals have an inclination to procreate, but the others just mate.  Unlike them, we marry; we participate in a practice imbued with the rational meaning of a procreative partnership.

So, although all of our natural inclinations pertain to reason, they don’t all pertain to reason in the same way.  Some are, so to speak, native-born citizens of reason, for example the inclination to know the truth, especially the truth about God.  Others are, so to speak, naturalized citizens of reason, for example the inclination to a procreative partnership.  The former are reasonable per se, whereas the latter are reasonable “by participation.”