 Finally, it is worth mentioning the extremely influential thought experiment described by Thomas Nagel (1974). According to Nagel, the physical facts about an organism and its perceptual systems are “objective facts par excellence – the kind that can be observed and understood from many points of view and by individuals with differing perceptual systems” (1974, 442). Nagel argues that even if we knew all the objective facts about a bat’s sonar system, we would still not know what it would be like to perceive using this system. Therefore, complete knowledge of the physical facts about a bat’s perceptual system would not yield knowledge of certain facts about a bat’s experiences; these facts can only be captured from a subjective perspective. Nagel does not argue against physicalism, but rather claims that we presently have no understanding of how it can be true. In Section 4.9, we shall outline a related approach to the knowledge argument which interprets it as a challenge not to physicalism but to a position called objectivism.