Full Idea
An organism only has conscious mental states if and only if there is something that it is like to be that organism.
Gist of Idea
An organism is conscious if and only if there is something it is like to be that organism
Source
Thomas Nagel (What is it like to be a bat? [1974], p.166)
Book Reference
Nagel,Thomas: 'Mortal Questions' [CUP 1981], p.166
A Reaction
It is hard to argue with this, but one should push on and ask what features of its consciousness make it such that there is a 'what it is like'. What is it like to have a subconscious mind, or be deeply asleep, or drive while daydreaming?