Anything you can think of is subject matter. Subject matter is anything you can think about, read about, talk about, imagine, or learn. Subject matter covers all the persons, places, things, facts, ideas, concepts, beliefs, theories, and processes that exist in the world. Subject matter is everything that has gone before us, everything that now exists, and everything that will exist. It exists out there in the world.
Humans look at nature.
They discover, research, and understand what is found in the natural world. Humans also look at themselves and others. They discover and invent things and ideas that help them to understand the human condition. In sum, everything that humankind has not invented or thought of can be called nature-based subject matter. We do not know with certainty why things in nature exist.