 In another important, related, challenge to functionalism (and, more generally, physicalism), Thomas Nagel (1974) and Frank Jackson (1982) argue that a person could know all the physical and functional facts about a certain type of experience and still not “know what it's like” to have it. This is known as the “Knowledge Argument”, and its conclusion is that there are certain properties of experiences — the “what it's like” to see red, feel pain, or sense the world through echolocation — which cannot be identified with functional (or physical) properties. Though neither Nagel (2000) nor Jackson (1998) now endorse this argument, many philosophers contend that it raises special problems for any physicalistic view (see Alter 2007, and, in response, Jackson 2007).