The Knowledge Intuition and the Inference to Physicalism’s Falsity

The knowledge argument has two parts. One says that physical knowledge is not sufficient for phenomenal knowledge. Call this the knowledge intuition (Stoljar and Nagasawa, 2004). The other says that the knowledge intuition entails the falsity of physicalism.

Thus described, the knowledge argument is not new with Jackson. Locke and other 18th Century British empiricists discussed the knowledge intuition. C. D. Broad gave a version of the knowledge argument in 1925. And other versions appear in more recent writings, such as Thomas Nagel’s 1974 “What is it Like to be a Bat?” What is distinctive about Jackson’s contribution?

Daniel Stoljar and Yujin Nagasawa (2004) answer this question in their introduction to a volume of essays on the knowledge argument. As they say, Jackson contributes at least two main ideas: his Mary example illustrates the knowledge intuition better than previous attempts; and he provides distinctive reasons for inferring physicalism’s falsity from the intuition. Let us take these points in order.

The Mary case divides the knowledge intuition into three claims:

• The complete-knowledge claim: before leaving the room, Mary knows everything physical.
• The learning claim: upon leaving, she learns something.
• The non-deducibility claim: if the complete-knowledge claim and the learning claim are true, then what Mary learns when she leaves the room cannot be a priori deduced (deduced by reason alone, without empirical investigation) from the complete physical truth.

Physicalists may deny the knowledge intuition. But the Mary case suggests that doing so requires rejecting the complete-knowledge claim, the learning claim, or the non-deducibility claim.

The cases discussed by Broad, Nagel, and others do not deliver this result. Consider, for example, Broad’s “mathematical archangel,” a logically omniscient creature who knows all the physical truths about various chemical compounds. Broad calls these truths “mechanistic” instead of “physical,” but the point is the same. On his view, the archangel would know all such truths but still lack phenomenal knowledge concerning, for example, “the peculiar smell of ammonia.” And Broad infers that physicalism (“mechanism”) is false. But what if the physicalist denies that the archangel would lack the relevant phenomenal knowledge? We appear to be at an impasse. By contrast, if the physicalist claims that, while in the room, Mary knows what it’s like to see colors, he must explain why she seems to acquire this knowledge when she leaves. The Mary case breaks the deadlock in favor of the knowledge intuition. Other illustrations of the intuition that precede Jackson’s have further drawbacks. For example, Nagel’s claim that humans cannot imagine what it’s like to be a bat raises distracting issues about the limits of human imagination, about which physicalism carries no obvious commitments. Mary’s fame is just.

To explain the second of Jackson’s distinctive contributions, it will be useful to explain some terminology and abbreviations. First, there is the distinction between the a priori and the a posteriori. A priori truths are those that are justifiable by reason alone, without empirical investigation. Logical truths provide clear examples. For example, one can figure out without empirical investigation that the following claim is true: if Socrates is mortal, then either Socrates is mortal or Socrates is fat. Compare the claim that Socrates is mortal. While we believe the latter claim to be true, reason alone does not justify this belief. Instead, we rely on experience—empirical investigation. So, while it is a priori that if Socrates is mortal, then either Socrates is mortal or Socrates is fat, it is a posteriori that Socrates is mortal. We may also speak of truths that are a priori deducible from other truths. For example, although “Socrates is mortal” is a posteriori, that same truth is a priori deducible from two other truths: “All men are mortal” and “Socrates is a man.” In other words, the latter two truths, taken together, a priori entail that Socrates is mortal.

Second, there is the notion of metaphysically necessary truths. A necessary truth is a truth that could not have failed to be the case. Logical truths again provide clear examples: “Either Socrates is mortal or it is not the case that Socrates is moral” is usually regarded as necessary. Contrast that truth with “Socrates is mortal.” The latter is not necessary. Truths that are not necessary are also known as contingent. Philosophers often distinguish between different strengths or kinds of necessity. For example, there is arguably a sense in which it is a necessary truth that pigs cannot fly like birds. But if the laws of nature were different, then perhaps pigs would be able to fly like birds. So, perhaps it is not metaphysically impossible that pigs should be able to fly like birds. A metaphysically necessary truth is a truth that is necessary in the strictest possible sense: a truth that holds not just because of contingent laws of nature. Saul Kripke (1972) famously argues that there are metaphysically necessary truths that are not truths of pure logic. Indeed, he argues that there are metaphysically necessary truths that are not a priori. For example, on his view, that water is H2O is metaphysically necessary but a posteriori. He recognizes that there could have been substances that resemble water—substances that share water’s superficial qualities, such as its taste and visual appearance—but with a different molecular structure. But, he argues, these substances would not be water.

Third, let us introduce some abbreviations. On Jackson’s version of the knowledge argument, the assumption that Mary knows the complete physical truth about the world does not guarantee that she will be able to figure out the complete truth about human color vision. His reasoning involves the idea of the complete physical truth. Call the complete physical truth P. P can be seen as a long conjunction of all the particular physical truths, which, according to Jackson, Mary learns from watching science lectures. What about the truths that, according to Jackson, Mary does not learn until she leaves the room? Those would be included in the psychological truths about the world. Call the complete psychological truth Q. Finally, consider what Stoljar and Nagasawa call “the psychophysical conditional”: if P then Q, where P is the complete physical truth and Q is the complete psychological truth. As we will see, part of Jackson’s reasoning can be understood in terms of his view about the psychophysical conditional.

We are now in a position to state the second of Jackson’s distinctive contributions to the discussion of the knowledge argument. This contribution concerns his inference from the knowledge intuition to physicalism’s falsity. His inference assumes that if physicalism is true then the complete truth about human color vision is a priori deducible from the complete physical truth. But here a problem arises: why accept this assumption? Consider the psychophysical conditional, if P then Q(again, P is the complete physical truth and Q is the complete psychological truth). As Jackson conceives of physicalism, physicalism entails that the psychophysical conditional is a priori. If he is right, then all truths about color vision would be deducible from P (the complete physical truth). But here physicalists have a natural, obvious response: why not instead characterize physicalism as a Kripkean a posteriori necessity, akin to water is H2O? On this characterization, the psychophysical conditional is metaphysically necessary but not a priori.

In later work, Jackson criticizes this response. His argument is complex, but the basic idea is simple enough. In a 1995 “Postscript,” he reasons as follows. Consider the argument:

H2O covers most of the planet.
Therefore, water covers most of the planet.

The premise necessitates, but does not a priori entail, the conclusion. But, Jackson asks, why is there no a priori entailment? On his view, there is no such entailment because the argument’s premise gives us only part of the physical story. It is also part of the physical story that H2O does the other things that water does, that is, that H2O plays the water role. Playing the water role includes such things as being a substance that occupies oceans and lakes, looks clear to us, has little or no taste, is referred to as “water”, etc. So, let us add the following premise to the argument displayed above:

H2O plays the water role.

Now, says Jackson, the premises do a priori entail the conclusion. Moral: “a rich enough story about the H2O way things are does enable the a priori deduction of the water way things are” (Jackson 1995, p. 413). Likewise, physicalism entails that “knowing a rich enough story about the physical nature of our world is tantamount to knowing the psychological story about our world” (Jackson 1995, p. 414). But if physicalism is true, P should provide just that: a rich enough story. Thus, Jackson concludes, physicalism entails the apriority of the psychophysical conditional after all.

Jackson’s argument is controversial. But in developing it, he fills an important lacuna in the knowledge argument and thereby improves on earlier versions. Others, too, have attempted to fill this lacuna. Most notably, David Chalmers (1996, 2003, 2004, and 2006a) has given sophisticated arguments to this end, which are partly inspired by Jackson’s argument.