In this chapter I defend the view that to know what it is like to experience a phenomenal property is just to be consciously acquainted with it, to experience it. Knowledge of what it is like is not knowledge that. It is not conceptual/propositional at all. It does not require thought, or the deployment of concepts. Nor is it knowledge what in the sense of, for example, knowing what time it is, or knowing what the positive square root of 169 is, which is also conceptual. And it is not some kind of know-how. It is, I will argue, simple acquaintance with, being familiar with, a phenomenal property. To know what a particular kind of experience is like is to be familiar with the phenomenal property or properties that characterize it; and to be familiar with such properties is just to experience them. Acquaintance is the fundamental mode of knowledge of phenomenal properties instantiated in experience, it is knowing what it is like; and all it requires is the experience itself.

Frank Jackson’s (1986) Mary is confined in a room in which the visual stimuli are all grayscale, so that her visual experiences have all been achromatic. Nonetheless, from books and television lectures, she has gotten all factual knowledge one can get from any source that pertains to the having of conscious visual experiences. It could indeed be “everything [factual] there is to know about the physical nature of the world,” based on “completed” science. But, Jackson writes, “[i]t seems … that Mary does not know all there is to know. For when she is let out of the black-and-white room or given a color television, she will learn what it is like to see something red, say” (1986, 291; emphasis Jackson’s).

As it is easier for a theorist to struggle with the most stubborn discrepancy than to abandon a favourite system, he has recourse to circumstances, which, though they leave precisely the same difficulty as before, are at least more complicated, and therefore better fitted to hide an inconsistency from the author himself, as well as from those whom he addresses.

Phenomenal knowledge is knowledge of what it is like to be in conscious states, such as seeing red or being in pain. According to the knowledge argument (Jackson 1982, 1986), phenomenal knowledge is knowledge that, i.e., knowledge of phenomenal facts. According to the ability hypothesis (Nemirow 1979; Lewis 1983a), phenomenal knowledge is mere practical knowledge how, i.e., the mere possession of abilities. However, some phenomenal knowledge also seems to be knowledge why, i.e., knowledge of explanatory facts. For example, someone who has just experienced pain for the first time learns not only that this is what pain is like, but also why people tend to avoid it.

Many agree that Frank Jackson’s Mary learns something when she leaves the black-and-white room and finally experiences color firsthand. There is less agreement about what her epistemic progress consists in. According to the new-concepts thesis, it consists at least partly in her acquiring phenomenal-color concepts, that is, concepts associated with knowing what it is like to see in color.

The knowledge argument is something that is both an ideal for philosophy and yet surprisingly rare: a simple, valid argument for an interesting and important conclusion, with plausible premises. From a compelling thought experiment and a few apparently innocuous assumptions, the argument seems to give us the conclusion, a priori, that physicalism is false. Given the apparent power of this apparently simple argument, it is not surprising that philosophers have worried over the argument and its proper diagnosis: physicalists have disputed its validity, or soundness or both; in response, non-physicalists have attempted to reformulate the argument to show its real anti-physicalist lesson.

In 1982 Frank Jackson published ‘Epiphenomenal Qualia’, in which he imagined the near-future scenario of a researcher into colour vision who is confined to a monochrome environment. Because it is the imaginary near future, she is able to compile all the scientific information about the physical goings-on that underpin colour vision

Mary in her black and white room knows all that physical science can teach us about the physical facts involved in colour experience. But it does not follow that she knows everything there is to know about these facts. The Russellian monist exploits this gap to defend a form of physicalism – in a very broad sense of that word. Unfortunately, recent developments in the grounding literature cast doubt on that strategy, or so I will argue.

Mary has new kinds of experiences when she leaves the black and white room. The change is akin to the difference between seeing black and white films and seeing films in colour. That much is common ground in the debate over the knowledge argument. This suggests that an ultimately satisfying reply to the argument on behalf of physicalism should base itself on a plausible view about the nature of the experiences she has for the first time on leaving the black and white room. It is, after all, the nature of these experiences that lies at the heart of the argument’s intuitive appeal. In this chapter, I offer an account of colour experiences and explain how it tells us physicalists what is wrong with the knowledge argument.

We all know the story of Frank Jackson’s Mary, locked in her black and white room and released to discover a world full of color. Less well known is the story of Uninformed Mary (hereafter U-Mary). Like Jackson’s Mary, she is locked in a black and white room and has been there her entire life. Unlike Jackson’s Mary, she does not have God-like knowledge of the physical facts of color and color experience. She has various books to read but she is indifferent to those that tell her about the colors. “What are they to me?” she mutters. “Never experienced them, never will.” One day, much to her surprise, she discovers that the door to her room is unlocked and opening it, she finds herself in another black and white room with a large reclining chair in the middle, a chair of the sort one might find in a dentist’s office. She goes over and sits in it. As she does so, clamps suddenly emerge from the chair arms holding her firmly in place, and a device descends from the ceiling that covers her head. The device contains probes that penetrate her skull as it descends. These probes are positioned so that they end up in her visual cortex and so stimulate it that she has a vivid hallucinatory experience of a red patch, an orange patch and a green patch on a white wall. A voice tells her to pay attention to the colors of the patches and informs her that the far left-hand patch is red, the middle one orange and the one on the right green. She is then told that she will be released into the outside world if she answers one question: Is red is more similar to orange or to green?

Frank Jackson’s knowledge argument is naturally viewed as a more precise version of Thomas Nagel’s argument in “What Is It Like to Be a Bat.” In particular, Jackson directed our attention to the challenge phenomenal knowledge poses for physicalism. Viewed this way, the knowledge argument focuses more on the question of “what it’s like to echolocate” than what it’s like to be a bat. The worry, to put it a more Nagelian way, is that the nature of phenomenal states escapes the objective picture of the world provided by our best sciences. I think there is another strand of Nagel’s argument that often gets ignored and that could be made clearer by a corollary to Jackson’s knowledge argument. This strand sticks a little closer to Nagel’s original question, and directs us to think about what it is like to be, or to have, a self. Just as in Jackson’s original argument, there can be much debate about what the implications of this corollary argument are – whether the implications are metaphysical or not; whether the argument even gets off the ground, etc. But I think the knowledge argument about the self is worth setting forth nonetheless, if only to get a little clearer about something else that might seem to be missing from the objective point of view.

In this chapter, we consider a computational-level theory of communication as Bayesian inference. We again illustrate the use of classical complexity analysis to assess the theory’s intractability. In addition, we show that parameterized complexity analysis can converge with theoretically informed intuitions about possible sources of this intractability, in this case based on the Gricean Maxims in pragmatic theories of communication.

This chapter discusses a set of possible objections to the tractability constraint on computational-level theories. Any of these objections may naturally arise when reading Chapter 1. It is an option to read this chapter directly after Chapter 1. However we believe that the responses to the listed objections can be best appreciated after having mastered the formal concepts and techniques in Chapters 2–7 and familiarized oneself with the conceptual arguments in Chapter 8.

In this chapter we introduce the classical complexity classes P and NP, where P consists of decision problems that are solvable in polynomial time and NP consists of decision problems for which yes-instances are verifiable in polynomial time. We show that the latter problem class contains many problems for which it is conjectured that no polynomial-time algorithm exists. We introduce the formal notions of NP-hardness and NP-completeness to show that a problem is as least as hard as the hardest problems in NP, and we describe why NP-hard problems are considered to be intractable (i.e., not computable in polynomial time). We explain how one can prove that a problem is a member of either P or NP, and how one can use the technique of polynomial-time reduction, introduced and practiced in Chapter 3, to prove NP-hardness and NP-completeness.