It’s the fact of subjective experience - the warmth of a campfire, the bitterness of lemon, the greenness of green. We’re essentially talking about consciousness here. The fact that there’s something it is like to be.

While nobody knows what consciousness is or how it comes about, what I mean by it is best captured by the philosopher Thomas Nagel in his aforementioned essay “What Is It Like to Be a Bat?”

Nagel argues that consciousness has an essentially subjective character, a what-it-is-like aspect. He states that “an organism has conscious mental states if and only if there is something that it is like to be that organism – something it is like for the organism.