As a philosopher, Descartes was known for his introspective nature and love of complex ideas.
In this first principle, we find the foundation of all his other philosophical endeavors: thinking about thinking itself.
This principle is like a philosophical onion, with layers upon layers of thought, until you finally get to the center where it says, "I think, therefore I am... not really, I just sit around thinking about thinking."
But seriously, this is where Descartes gets his reputation as a father of modern philosophy, so don't @ me.