Thomas Nagel in his essay does not define consciousness per se, but rather gives numerous properties of consciousness. The most important property by far for his argument, is that if and only if an organism is conscious, by having conscious mental states, there exists also a subjective character of experience for that organism. (p. 436)
In addition to this core property, Nagel also claims that consciousness is a very common phenomenon specific to animal life on Earth (p. 436). His support for the commonality of the phenomenon is surprising, considering that he admits later in the same phrase that “it is very difficult to say in general what provides evidence of it”. Nevertheless, for the core of his argument presented in the next exercise it would be enough to accept that individual humans posses consciousness, and for understanding the intuition behind it it is useful to extend that assumption to other complex and related forms of life on Earth such as echolocating bats. Nagel also posits under strikingly strong terms that consciousness exists in other parts of the universe: “No doubt it occurs in countless forms unimaginable to us, on other planets in other solar systems throughout the universe”. His lack of doubt here can be interpreted as rhetoric, since we cannot be so sure of such a fact given that we don’t know exactly what provides evidence of consciousness in general nor can we imagine the “countless forms unimaginable to us”.
In regards to whether consciousness affects or does not affect the behavior of an organism, Nagel prefers not to take a specific position and to just acknowledge the existence of both possibilities (p. 436), most likely since it does not seem to affect his argument regarding the problems that consciousness raises for accepting physicalism.
- What Is It Like to Be a Bat?, Thomas Nagel, The Philosophical Review, Vol. 83, No. 4. (Oct., 1974), pp. 435-450