BonJour begins by presenting one of the main arguments present in Thomas Nagel's paper What Is It Like To Be A Bat?². That argument is that an organism has conscious mental states if and only if there is something it is like to be that organism - something it is like for the organism. By this Nagel means that such an organism has a subjective point of view. He presents the example of a bat, which presumably has at least some sort of conscious experience, and so he concludes there is something it is like to be a bat. Of course, bats perceive the world through sense organs drastically dissimilar to our own (for example, they perceive spatial relations by a form of echolocation rather than vision), and so it is reasonable to assume their experience is vastly different from ours.

Nagel claims that we humans could never come to know what it is like to be a bat (not to be confused with what it is like for a human to have bat-like experiences or for a human to imagine itself as a bat), even if we came to know every relevant detail about the physiological and neurophysiological makeup of bats. He bases this on the claim that physical science (in an attempt to reduce phenomena to purely physical terms) works within an objective point of view, and since what it is like to be a thing (for example, a bat) is fundamentally connected to a subjective point of view, any move toward a physical explanation will be a move away from what it is like to be that thing.

In other words, physical science can never discover the facts of what it is like to be a thing, and so there are apparently non-physical facts. Therefore it is implied that perhaps physicalism is false, or at least that it is beyond us how it could be true (though, as BonJour points out, Nagel leaves open the possibility that we could have reason to believe physicalism is true even if we lack an understanding of how it could be so).

According to BonJour, Nagel's basic argument about the inability to reduce a subjective point of view to objective physical terms is essentially sound. However, BonJour suggests that many of the other issues raised in Nagel's paper may be irrelevant to or even inconsistent with the argument BonJour wants to focus on. For example, he points out that Nagel's formulation of the argument leaves open the possibility for someone to object that what is lacking from a physical account is not knowledge of facts, but rather knowledge of what it would feel like from the inside to be a bat, something which might not be expected of a physical account even if physicalism were true. Therefore, BonJour's aim later in his paper is to present a reformulated argument which he believes avoids such objections.

Physicalism and Jackson
Before presenting his own argument, however, BonJour also lays out a brief version of the famous Knowledge Argument against physicalism presented by Frank Jackson in a paper titled Epiphenomenal Qualia³. Jackson defines physical information about an organism as information about the functional role of the processes that go on in the organism (including how they relate to the external world). He argues in the first section of the paper that we experience phenomenal qualia, and yet a physical account leaves these qualia out. That is, having all the physical information about an organism does not give all the information about an organism. Therefore, there is supposedly non-physical information, and so physicalism must be false.

To argue this point, Jackson presents a thought experiment. The thought experiment involves a genius scientist named Mary who has lived her entire life in a completely black and white room (supposedly her skin would be dyed black and white, and so forth) with access to books painted black and white and television shown in black and white. Through reading books and watching television lectures, Jackson explains, Mary learns all there is to know about the neurophysiological processes involved in human vision (for example, what happens when a person looks at a ripe tomato or clear sky and uses the terms 'red' and 'blue'). Her information includes things like how certain wavelengths of light stimulate various parts of the human eye and how this leads to certain brains processes which eventually lead to output signals causing a person to say something like "the sky is blue".

Jackson then asks what would happen if Mary were to one day leave her black and white room (or simply get a color television) and enter the world of color. He argues that upon viewing a red tomato or a blue sky, she would learn a new fact (what it is like to see a red or blue object) and so all there is to know about the neurophysiological processes involved in human vision does not encompass all there is to know about human vision. Since Mary had all of the physical information, there must be non-physical information involved, and so Jackson concludes that physicalism is false.

BonJour suggests that Jackson's argument gets more directly at the point he wishes to make than Nagel's argument, but he feels it is not as decisive as it should be, since it is still open to objections that avoid the main issue involved. BonJour concentrates on two objections offered by Paul Churchland in Reduction, Qualia and Direction Inspection of Brain States. The first objection is that what Mary learns upon entering the world of color is not knowledge of facts or (propositional) truths about human mental states, but knowledge in the sense of having a representation of color in some prelinguistic or sublinguistic medium of representation of sensory variables. Churchland claims that knowledge of facts or truths is all a physical account can be expected to supply, and since Mary doesn't gain knowledge of facts or truths, but something else, Jackson's argument fails.

BonJour suggests that this objection by Churchland is wrong, but that the mistake is hard to demonstrate within the context of the Mary example. So BonJour's own example, to be presented later, will need to show that there are actual facts or truths that Mary learns upon entering the world of color, and what those facts or truths are.

The second objection Churchland makes against Jackson's argument is that if Mary's knowledge of the physical processes involved in vision was in fact complete, she could learn to employ the concepts of this completed neuroscience in introspection, and so imaginatively extrapolate from her black and white experience to the experience she would have if she had the neurophysiological states corresponding to color experience. That is, if her physical knowledge was complete, she could actually come to know what it is like to see something red, even if she never leaves the black and white room. BonJour feels that Churchland's objection is again mistaken, and in this case irrelevant to the central issue as well. What is needed is a formulation of a new argument adapted from Jackson's and Nagel's arguments which will avoid these objections, and other irrelevant side issues that have blurred the focus of the main point.

BonJour's Attack on Physicalism
BonJour claims to present just such an argument in the second section of his paper. He proposes that the reader imagine a brilliant Martian scientist which comes to Earth to study humans. Being a Martian, it has different sensory organs than humans (it lacks eyes, for example), but they are sufficient to arrive at a complete knowledge of the purely physical phenomena that go on in humans. Suppose eventually this Martian scientist does come to an ideally complete knowledge of the neurophysiological facts relating to human color experience (including causally defined functional roles). Would it thus know every fact or truth about human color experience?

The answer to this question, according to BonJour, is negative. To support this he proposes a thought experiment. Imagine a human looking at grass and having an experience of the phenomenal property it calls green, and then later looking at a fire hydrant and having an experience of the phenomenal property it calls red. Imagine as well that the Martian scientist measures all the neurophysiological processes (including causal relations to other states) taking place as the human has these experiences. In addition, BonJour grants that the Martian has some concept of red and green on its own (perhaps it perceives color through hearing music), and even somehow knows that each of the two neurophysiological states it measures in the human corresponds to one of those color experiences (keeping in mind that it lacks eyes and so cannot simply look at the objects and perceive what the human perceives).

Given all of this, it follows that there is a pair of propositions which are true about the human. Those propositions are that neurophysiological state X corresponds to color experience Y, and that neurophysiological state A corresponds to color experience B (as opposed to X corresponding to B and A corresponding to Y). BonJour asks whether the Martian scientist, given all of the above information, could discover the fact that one pair of propositions is true rather than their opposites (in this case, that the neurophysiological state involved in looking at grass corresponds to green experience, and the neurophysiological state involved in looking at a fire hydrant corresponds to red experience). If physicalism were true, the Martian scientist would have all of the relevant facts or truths, and so could discover the fact in question. If the Martian cannot discover the fact in question, physicalism must therefore be false.

BonJour argues that there is no way the Martian could discover that fact. To do so on the basis of nothing but the physical information it had access to, the concepts of red and green would have to either be already present in the neurophysiological account or else definable on the basis of the concepts that are present in that account. BonJour discounts the first of these possibilities since neurophysiology obviously does not directly invoke the idea of phenomenal color. He argues that the other possibility is no better because color concepts are primitive or indefinable. In addition, he suggests as an alternate reason for discounting the possibility of defining color concepts on the basis of neurophysiological terms that the view is less plausible than the supposedly discounted (or at least not seriously defended) phenomenalist view that physical object concepts are definable in purely sensory terms. Therefore, the Martian could not discover the fact, and so BonJour concludes that physicalism must be false.

This account presumably avoids the objections and irrelevancies raised by the arguments of Nagel and Jackson. Firstly, it deals with knowledge of facts or truths (specifically, the pair of propositions specified above) rather than what it feels like from the inside to be something, and so it avoids the obvious objection to Nagel's account. In the same way, it avoids Churchland's first objection to Jackson by focusing on the learning of facts or truths rather than the learning of what it is like to have a color experience. Also, BonJour avoids Churchland's second objection since it is irrelevant to his argument. The Martian scientist already has the concepts of red and green at his disposal, so it is not a matter of extrapolating what those experiences are like (whether or not such an extrapolation is even possible), but instead a matter of facts or truths. So if BonJour's essential argument is correct, it seems he has avoided the problems that beset Nagel and Jackson, and has presented an apparently definitive argument against phyicalism.

Objections to BonJour's Argument
There are two possible responses on the part of the physicalist which BonJour considers at the end of the second section of his paper. The first of these responses is that there are two sorts of facts which would not be in a complete physical account even if physicalism is true: facts about a thing's function (for example, one can have all the physical facts about a certain object without knowing that it is a chair because being used as a chair is a function for human beings, not part of an objective physical description) and facts about conventional or arbitrary classification relative to human needs (for example, one can have all the physical facts about certain molecules without knowing that they are hot or cold, by common sense meanings of those terms). It is supposedly implied that a physicalist making this response would consider the facts unavailable to the Martian to fall into one or both of these categories and so be exempt from a physical account even if physicalism is true.

BonJour points out that the two types of exempt facts are in fact exempt because they have to do with relations and only one of the relata is known (that is, the intrinsic or non-relational properties of only one of the relata are known). However, according to BonJour phenomenal experience is an intrinsic (not relational) property of a person, and so it is not one of these types of exempt facts.

The second possible physicalist response that BonJour considers is an appeal to a certain idea about relations between conceptual schemes. The idea is that the Martian does actually know the fact in question (the true pair of propositions connecting each neurophysiological state with the appropriate color experience) indirectly, because it has in its body of knowledge certain propositions which describe the same facts described by the true pair of propositions; however, the Martian cannot, even in principle, tell that this is the case.

The idea involved in this claim is basically that descriptions of the same fact in incommensurable conceptual schemes need not be logically or recognizably equivalent to each other. An analogous example which BonJour mentions would be picking out a certain color by specifying "Joe's favorite color" (where you do not know what color is Joe's favorite) and then indirectly or accidentally (and unknowingly) picking out the same color by specifying "the color normally experienced when viewing objects of a certain type (grass, say)".

BonJour's response is that not all property specifications can be indirect or accidental in this way, since some are often specified in a way that captures their intrinsic character. The properties of having certain color experiences, as well as the physical properties, are specified in this intrinsic way. Any two such property specifications which are not logically or recognizably equivalent must therefore be two distinct properties. That is to say, the neurophyiological knowledge the Martian has and the facts he allegedly does not have are specified in an intrinsic way and so it cannot be that they are simply the same thing specified under different conceptual schemes. In addition, BonJour points out, even if they were the same property, that property would thus have an inherent internal duality or complexity, and so the Martian would not be able to know that the experiential aspect of the property was present when the neurophysiological aspect was. Therefore, the physical account would still be false.

Intentional and Conscious Mental States
After thus establishing his revised argument against physicalism, BonJour spends the last two sections of the paper attempting to extend the basic argument to mental states that have intentional content (in the third section) and to conscious mental states (in the fourth section). For the former issue, he focuses on the simple example of a person thinking about something, say dogs in general, and asks whether the Martian could tell on the basis of a complete neurophysiological account that the person was thinking of dogs. BonJour claims that if physicalism is true, the Martian should be able to know what the person is thinking about because the thinker can pick out what they are thinking about from the inside alone (that is, thought content is a property internal to the person). BonJour defends this claim by considering and refuting two objections from the philosophy of language which dispute the idea that thought content is internal.

The first such objection comes from Hilary Putnam in The Meaning of Meaning, who states that a thinker does not need a clear conception of dogs to think about them, but that what is required is employing the term "dog" in thinking, where the meaning of that term is fixed by the relevant group of experts. Since experts are external to the thinker, the Martian should not be expected to determine what the thought is about based solely on the thinker's neurophsiological information.

BonJour points out that this implies that the thinker's conception of dogs will be partially indeterminate, since it is possible there are non-dogs that the thinker could not distinguish from dogs. However, he replies that even if thoughts of dogs are indeterminate, the Martian should still be able to tell what the indeterminate content is (insofar as the thinker can do the same). In other words, there is at least something which is internal to the thinker, and which, if physicalism is true, the Martian should be able to access.

Another philosophy of language objection BonJour considers is based on the causal theory of thought content which states that what a person is thinking about is not determined by internal states alone, but depends on external causal relations (including the causal history of words the person employs). BonJour concedes that this may be partially right, since part of what makes a person's thought be about dogs and not indistinguishable Twin-Earth dogs is the causal relation to Earth dogs rather than Twin-Earth dogs by the causal history of the word. However, BonJour points out, this does not imply a completely external account of thought content because such an account would imply that a person would have no grasp of what they are thinking about since they have no access to the causal relations. Therefore, there must be something internal, and again it is this which the Martian should be expected to know if physicalism is true.

The question, then, is whether the Martian actually could know what a person was thinking about based upon the neurophyiological information. BonJour claims that it could not by an argument parallel to his earlier argument that the Martian could not discover the facts relating to the correspondence of certain neurophysiological states with certain color experiences. He points out that even if the internal content of a thought about dogs, say, is somewhat indeterminate, it at least includes certain rough features of being a medium-sized, hairy animal of a distinctive shape. Even if one grants the Martian an independent conception of these features and the knowledge that this is one possible content for the thought in question, BonJour argues that the Martian could not know that the thought in question has this content.

This is because the pieces that make up the content (hairiness, barking, etc.) are not explicitly present in the neurophysiological account, and it is extremely implausible that they could be defined by what is present in that account. The Martian will thus not have access to those pieces of the content based on the neurophysiological account, and so there is nothing it could use to discover that the thought in question is about dogs.

Before continuing on to the subject of conscious mental states, BonJour considers one further possible response on the part of the physicalist based on what he calls a coherence theory of conceptual content, which states that concepts are defined entirely by their formal structure and inferential relations to each other. On this view, anything that realized the appropriate structure would represent the relevant content. So the Martian, by knowing the causal structure of the thinker's neurophysiological states would supposedly be able to identify the contents corresponding to those states.

BonJour suggests however that having the appropriate structure is only a necessary condition for representation, not a sufficient condition, since a system of states could accidentally obtain that structure without representing anything at all. Hence, the Martian could not know that a person is thinking about something on the basis of formal structure alone, since the structure could be an accident. In addition, BonJour claims that the coherence theory of conceptual content is implausible since it is implausible that inferential structure alone could even identify a particular set of concepts. Rather, it is entirely possible that many distinct concepts have indistinguishably similar inferential structures. Therefore, BonJour concludes, it is very implausible to think that the Martian could tell what a person was thinking on the basis of neurophysiological information alone, and so physicalism about intentional mental states is very implausible.

Finally, in the fourth section of his paper, BonJour extends his Martian scientist argument to conscious mental states. He points out that there are many neurophysiological states which are certainly not conscious (such as those that control breathing), and asks whether the Martian could distinguish conscious from non-conscious states on the basis of neurophysiological information alone. If not, conscious mental states are not physical. BonJour argues that indeed the Martian is unable to pick out conscious states, and that is because (once again paralleling his original argument) consciousness is not explicitly mentioned in a complete neurphysiological account, nor is it plausibly definable in the terms of such an account. He states that no one would seriously suggest otherwise to either of these points. Therefore, the Martian would have nothing at its disposal to distinguish conscious states from non-conscious ones. BonJour concludes his paper by suggesting that this, along with the earlier arguments, demonstrate that a complete neurophysiological account is significantly incomplete, and so physicalism must be irredeemably false.

Subjective Physical Facts
I believe that while BonJour's objection to physicalism may be initially powerful, closer inspection reveals that it is working under a faulty assumption which renders its conclusions less forceful than they seem. My response to BonJour hinges on a topic he barely touches on in his paper: what is the physical, and what does the truth of phyicalism actually imply? I aim to show that BonJour is working under a mistaken conception of the physical, and by reconsidering that conception, I think it can be shown that physicalism is not as bad off as BonJour would have us believe.

First off, it will be helpful to be more explicit about the concept of physicalism used by BonJour; and since Nagel and Jackson laid the framework for his article, it will also help to examine their conception of physicalism. Nagel speaks of physicalism as fundamentally being a process of reduction, and one that strives toward complete objectivity (description that does not rely on a single point of view). Since the realm of the subjective is based on a single point of view, a physical story apparently could not account for it, and so physicalism is an incomplete reduction. He mentions in a footnote that he does not attempt to explicitly define the term physical in the paper, but that whatever else may be said of the physical, it has to be objective. Jackson appears to take a different approach. Roughly, he defines physicalism as the thesis that physical information is the only information. His idea of physical information is the information provided by and expressed in the terms of physics, neurophysiology and similar sciences. So physicalism under his view would imply that any information could be expressed in the terms of these sciences.

BonJour never explicitly states what he means by physicalism, but he seems to be working under a definition that combines and modifies those of the previous authors. Like Jackson, he certainly seems to view the physical as that which can be put into the terms of science. He also seems to adopt Nagel's idea that the physical is objective. His addition is a focus on facts or truths. A physical account is one of facts or truths, which are what BonJour considers objective (as opposed to what it feels like to be something, as BonJour points out in his brief criticism of Nagel). Thus, BonJour's conception of physicalism seems to be the thesis that all facts or truths (as objective things) can be put into the terms of science. His Martian story sets out to provide a counter-example to the claim by providing a fact that cannot be put into those terms.

Some brief comments of elaboration are important to focus these ideas. First, BonJour is speaking of a completed physics and neurophysiology in his remarks. No one would claim that the only physical facts are those that can be put into the terms of contemporary physics for certainly new developments and discoveries will arise for which new terms will be created to describe and explain them. Second, BonJour's paper focuses on knowledge of facts or truths (what it is the Martian would come to know by having a complete scientific understanding), but implicit in this is the idea that all facts or truths can be put into the terms of science if physicalism is true (because to gain knowledge by science obviously means working with the terms of science). Finally, BonJour's example of the Martian deals with a fact or truth which includes as a part of its content a subjective thing (color experience).

I believe that BonJour's conception of the physical is problematic, and I will aim to present a better conception, developed by Max Deutsch⁴. I will argue that as a result of this clarified idea of the physical, the idea of a subjective, physical fact will not be a problem. Finally, I will aim to show that the implications of BonJour's example, when viewed with these distinctions in mind, are not problematic for physicalism.

The fundamental problem with BonJour's conception of the physical is that it seems to be linguistic (that is, based on the language of science) rather than metaphysical. The conclusion of BonJour's argument (as well as Jackson's) is merely a conclusion about the failure of a certain language to capture or express certain facts or truths. It is not a conclusion about the failure of a metaphysical thesis, which is what most people would agree the physical (and thus physicalism) really is.

Rather than trying to give a perfect and precise definition of the physical, Deutsch presents a rough sketch (which I adopt here as sufficient at least to respond to BonJour's paper) of what it is for something to be physical by looking at a certain type of metaphysical relation, that of being the same sort of ontological type as another thing, where that is the case if and only if those two things can stand in causal relations to one another.

BonJour would certainly concede that fire hydrants are physical things. This establishes an example of a physical thing, and from this a rough idea of the physical can be developed by saying that anything which is of the same ontological type as a physical thing, is itself a physical thing. The question then is whether the fire hydrants are of the same ontological type as the experience involved in seeing a fire hydrant. The answer is that they are, for it seems obvious that a fire hydrant can stand in a causal relation to the experience of seeing a fire hydrant (the presence of a fire hydrant is, in the case of normal vision, enough to produce, that is cause, such an experience). Therefore, under this metaphysical concept of physical, color experiences are physical because they are of the same ontological type as an admittedly physical object.

In order to show how this distinction applies to BonJour's paper, it will help to consider the other aspect of BonJour's conception of physicalism: the objective/subjective distinction. There are certainly many different ideas of what the difference between objective and subjective is, but I think the one relevant to the topic at hand is as follows. Objective facts (or knowledge) are those that can be known from more than one perspective or point of view, that is those that can be known in more than one way (for example, one can know about the number of legs tigers have by asking a reliable zoologist, watching a documentary about tigers, or reading a reliable book about tigers).

Conversely, subjective facts (or knowledge) are those that can be known only from a specific, single perspective or point of view, that is those that can be known in only one way - by experiencing them. An example of this would be what it is like to be something, what it is like to see red, and so forth. This distinction would presumably be acceptable to BonJour, and it seems to fit the intuitions raised by the terms objective and subjective.

Now, in considering the nature of objective and subjective facts under the light of the metaphysical conception of physical given above, it becomes clear that there is nothing problematic or contradictory in the notion of a subjective, physical fact. It would simply be a fact about an object of the same ontological type as other obvious physical objects, which could be known only one way, by experience. Thus, in the case of Jackson's knowledge argument, what it is like to experience color is a subjective, physical fact which Mary did not have while inside the black-and-white room (assuming, as seems plausible, that knowledge of objective, physical facts does not imply knowledge of subjective, physical facts; if it did, then Mary would know what it is like to experience color and the Martian would know which fact obtains regarding the pair of true propositions). What she had was all of the objective, physical facts (which is what science provides as an objective enterprise, as Nagel points out), but not all of the physical facts.

Extended to the case of BonJour's Martian example, the Martian would not have the fact in question because, as pointed out above, that fact includes subjective content. That BonJour granted the Martian an antecedent conception of red and green is actually misleading. It tricks the reader into expecting that because the Martian is familiar with the subjective content, it would have all the necessary ingredients and so be able to determine which pair of propositions is true.

Instead, as BonJour himself points out by mentioning that colors are not explicitly mentioned nor based upon the language of science (neurophysiology specifically), knowledge of the objective, physical facts does not give one knowledge of the subjective, physical facts; and that means it fails to grant not only the concepts of red and green itself, but also that the objective, physical facts lead to those specific contents rather than another. Thus, it seems obvious under this distinction that the Martian would not be expected to know which pair of propositions is true.

This avoids BonJour's argument by denying a premise that BonJour takes for granted in his paper due to his taking for granted a linguistic conception of the physical. Rather than denying that there are facts the Martian doesn't know, I deny that the Martian, in virtue of having a completed science (physics, neurophysiology, etc.), would have all of the physical facts. Instead, he would have all of the objective, physical facts, but still be missing the subjective, physical facts. This idea could be extended to his sections on intentional and conscious mental states, since what he calls the internal factors are subjective factors and so not expected to be known from the Martian's scientific (objective) knowledge. BonJour's Martian scientist argument (and so in addition, Jackson's knowledge argument) is thus not at all problematic for the physicalist thesis.

¹ BonJour, Lawrence - What Is It Like To Be A Human (Instead Of A Bat)?

² Nagel, Thomas - What Is It Like To Be A Bat?

³ Jackson, Frank - Epiphenominal Qualia.

⁴ Max Deutsch - Subjective Physical Facts.