If second nature is what comes natural, then what is first nature?

Answer 1

Your original true nature, the unbounded , formless, spaceless, all pervading, everlasting, timeless, etc..
This is to be only understood as what is being referred to in relation to 'first' and the 'second' (since the second is reasonably acceptable, at least logically ) !
The whole purpose of spirituality is to open a possibility to release you from this 'second' nature, by going beyond nature, so that you evolve into your 'first' nature !
Best wishes, Let us make it happen !

Answer 2

The phrase "second nature" is used to over-describe an action that, as you said, comes naturally to someone. It comes so naturally that one would assume that that person was born knowing how to perform that action. But since they were not, since the action was in fact learned, then it is second to a nature which is instinctual. When one knows something instinctually, we call it a part of our nature. For instance, one would say that breathing is not learned. We do not have to think or watch someone else breathe to understand that we must do that same. This is different from say riding a bike very well. No matter how well one learns to ride that bike - well enough to become "second nature" - it was still required for that person to LEARN to do it.

Thus the difference between 2nd nature and 1st nature is that the former is learned, while the latter is instinctual.