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.