My subjective reality based on what I feel is real. There is more to reality that matter and energy, something exists that can not be fully explained by physical laws.
Material realism is a philosophy that has been gaining strength alongside the Scientific Revolution. Carl Sagan of Cosmos expressed this philosophy concisely in his opening The Cosmos is all there is, was or ever will be. The idea is that everything is made up of matter and energy and can be fully explained by physical natural laws.
Descartes tried to express the idea that there is something more to reality than the natural laws he was trying to understand and explain, in his famous expression " I think therefore I am"
While acknowledging the reality of the physical laws Descartes maintained that there was a reality outside of the physical laws.
The debate is can feeling be explained completely by physical natural laws or is Descartes correct that there is something beyond physical laws. This has implications about Human essence. Do Human truly exists as separate self determining entities or are humans simply extensions of universal natural forces.
I feel therefore I am is a statement that humans do exist as separate self determining entities.