The Knowledge Argument

Core Idea

Frank Jackson's knowledge argument imagines Mary, a scientist with complete physical knowledge of color vision who has lived her whole life in a black-and-white room. Upon seeing red for the first time, Mary learns something new: what it's like to see red. This suggests consciousness involves non-physical knowledge not captured by physics.

Explainer

You already know that phenomenal consciousness involves qualitative experience — the felt character of seeing red, tasting coffee, or feeling pain. These phenomenal properties (qualia) are characterized by what Nagel called "what it's like": there is something it is like to be you seeing red, something that goes beyond the physical description of wavelengths hitting your retina. Jackson's argument takes this seriously and turns it into a formal challenge to physicalism.

The thought experiment runs as follows. Mary is a brilliant neuroscientist who has spent her entire life in a room containing only black-and-white objects, screens, and books. She studies the complete physical science of color vision: every wavelength, every retinal cone's response, every neural pathway, every behavioral disposition humans have when they see colored objects. She knows, in exhaustive physical detail, everything there is to know about what happens when someone sees red. Then one day she leaves the room and sees a ripe tomato for the first time. Does she learn anything new? Jackson says yes — she learns what it is *like* to see red. Before, she knew all the physical facts. After, she knows something she did not know before. Therefore, not all facts are physical facts. The argument's structure: (1) Mary has complete physical knowledge before release; (2) she learns something new upon release; (3) therefore physicalism — the claim that physical facts exhaust all facts — is false.

The ability hypothesis is the most important physicalist response. Daniel Dennett, Lawrence Nemirow, and David Lewis argue that Mary does not gain propositional knowledge (new facts) — she gains know-how: the ability to recognize, remember, and imagine red experiences. On this view, "knowing what it's like" is not knowing a fact; it is acquiring a skill. The gap between Mary's pre-release and post-release state is a gap in abilities, not in information. This sidesteps the dualist conclusion by denying that new knowledge of facts has occurred.

A second response is the phenomenal concepts strategy: Mary already knew all the physical facts, but she lacked the phenomenal concepts — the first-person, experience-grounded concepts — needed to think about those facts in the way one thinks about them from the inside. When she sees red, she acquires a new *way of thinking about* the same physical states she already knew about. On this view, there is no new fact, only a new conceptual perspective on an old fact. The challenge for this view is to explain why phenomenal concepts give us a different "grip" on reality without admitting that the reality they reveal is non-physical.

What makes the knowledge argument so enduring is that it isolates the explanatory gap: even if we fully understood all the physical mechanisms of color perception, there would remain a question that physics does not seem to answer — why does neural state N feel like *this*? Jackson himself later retracted the anti-physicalist conclusion, arguing Mary learns only new modes of presentation of old facts. But the thought experiment continues to set the terms of debate, because it makes vivid the intuition that first-person, phenomenal knowledge is not simply derivable from third-person, physical description.