Among philosophers, it is often suggested that there is some link between conceivability and metaphysical possibility. Most commonly, this suggestion takes the form of the following principle: If P is conceivable, then P is metaphysically possible. This principle is sometimes qualified so as to specify that the link in question is defeasible. In other words, if P is conceivable, this provides defeasible evidence for the claim that P is metaphysically possible.
A stronger form of this alleged link between conceivability and possibility has been proposed along the following lines: If P is conceivable, then P is metaphysically possible. Furthermore, if P is not conceivable, then P is metaphysically impossible. The latter half of this stronger principle can also be qualified in terms of defeasible evidence.
These principles underlie two key arguments against physicalism in the philosophy of mind:
It is conceivable that all of the actual physical facts obtain in the absence of any mental facts.
If it is conceivable that all of the actual physical facts obtain in the absence of any mental facts, then it is possible that all of the actual physical facts obtain in the absence of any mental facts.
Therefore, it is possible that all of the actual physical facts obtain in the absence of any mental facts.
This is the so-called “zombie argument” against physicalism.
The second argument runs as follows:
If mental states/properties are physical states/properties, then it is possible that there are borderline cases of mental states/properties.
It is inconceivable that there are borderline cases of mental states/properties.
If it is inconceivable that there are borderline cases of mental states/properties, then it is not possible that there are borderline cases of mental states/properties.
Therefore, it is not possible that there are borderline cases of mental states/properties.
Therefore, mental states/properties are not physical states/properties.
This is the so-called “vagueness argument” against physicalism.
Premise 2 of the zombie argument and premise 3 of the vagueness argument are both supported by the conceivability principles described above. Consequently, in order to evaluate these arguments, we should evaluate the conceivability principles that support them.
Turning first to the weaker conceivability principle, one potential motivation for accepting that conceivability is evidence for metaphysical possibility is its intuitive plausibility. It seems to us that what we conceive of being the case is what is metaphysically possible. However, how should we understand problematic cases such as the apparent conceivability of a world where water is not composed of H20 (it has been persuasively argued by Saul Kripke that such a world is metaphysically impossible)? In such cases, Kripke and others have suggested, what we really take to be conceivable is a world where some watery stuff is not composed of H20, rather than a world where water is not composed of H20. Consequently, the apparent contingency of water’s identity with H20 can be explained away in terms of a world where the contingent properties we normally associate with water are not instantiated by H20. According to this explanation, conceivability is a genuine guide to metaphysical possibility and metaphysically impossible worlds are inconceivable.
However, what explains why we take certain possibilities to be conceivable at all? Without offering a general account, the following hypothesis suggests itself: When we normally apply two empirical concepts on the basis of independent evidence (where the evidence that leads us to apply one concept is not necessarily associated with evidence that leads us to apply the other concept in the same situation), we can conceive (or take ourselves to conceive) of a world in which the evidence that leads us to apply one concept is present in the absence of the evidence that leads us to apply the other. For example, since our H20 concept is applied on the basis of H20’s presence while our water concept is applied on the basis of some watery stuff’s presence, we can conceive of a world where some watery stuff is present in the absence of H20. Likewise I can conceive of a world where a journalist at the Daily Planet is present in the absence of a flying superhero with a red cape. This latter case gives rise to the apparent contingency of Clark Kent’s identity with Superman.
The hypothesis described above provides a satisfying explanation for why we take conceivability to be a guide to metaphysical possibility: If the evidence that leads us to apply one empirical concept is necessarily associated with the evidence that leads us to apply another concept, in most cases, we will apply both concepts in the same epistemic situations. Consequently, if we recognize that we do not apply both concepts in the same epistemic situations, this is a good reason to conclude that their referents are only contingently correlated. However, this principle runs into trouble when we consider the case of mind-brain identity.
When we apply our C-fiber concept and our pain concept, we do so on the basis of different evidence. We apply our C-fiber concept on the basis of the presence of C-fibers. By contrast, we apply our pain concept, directly, on the basis of our experience of pain and, indirectly, on the basis of the behavior of other organisms. According to the hypothesis under consideration, this will entail that we can conceive of a world where pain is present in the absence of C-fibers. But unlike ordinary cases where we apply our empirical concepts, our C-fiber and Pain concepts are applied on the basis of two radically distinct forms of epistemic access to reality. Specifically, our Pain concept is applied on the basis of direct experiential acquaintance while our C-fiber concept is not. For this reason, even if the evidential bases for applying these concepts were necessarily associated, we would still not expect them to be applied in the same situation. This is because, in one situation, we may be directly acquainted with pain without being in a situation where we would apply our C-fiber concept. Likewise, we could be in a situation where we would apply our C-fiber concept without being directly acquainted with pain. Because such cases are possible, the fact that we do not apply our Pain and C-fiber concepts in all of the same situations is not a good reason to conclude that their referents are only contingently correlated. Consequently, if the hypothesis presented above is accepted, we have a good reason for rejecting premise 2 of the zombie argument.
Turning now to the stronger conceivability principle, one motivation for accepting that inconceivability is a guide to the metaphysical impossibility of borderline cases is the simply fact that, normally, if a concept is vague, we can conceive of situations in which a relevant borderline case of its referent is present. What explains why this is true? Without offering a general account, the following hypothesis suggests itself: When we have a competent grasp of a given empirical concept, we can conceive of the sorts of situations in which the evidence that leads us to apply that concept is present. If an empirical concept is vague, it will normally be the case that the evidence that leads us to apply that concept can come in greater or lesser degrees. Consequently, if we competently grasp a vague empirical concept, we should be able to conceive of a situation in which the relevant evidence is present to such a degree that we recognize a borderline case of that concept’s referent.
The hypothesis described above provides a satisfying explanation of why we take the inconceivability of borderline cases as evidence for their metaphysical impossibility: If we can conceive of the sorts of situations in which the evidence that leads us to apply a vague empirical concept is present, in most cases, we will be able to conceive of situations in which that evidence is present to greater or lesser degrees. If we are unable to do so, this is a good reason to conclude that a given concept is sharp. However, this principle runs into trouble when we consider the case of consciousness.
When we conceive of conscious experiences, we apply our “Consciousness” concept on the basis of the phenomenal character of our experience. Crucially, we do not use this concept to refer to the phenomenal character of our experience. Rather, when we recognize the phenomenal character of our experience, this is our evidential basis for concluding that we are having a kind of conscious experience. Unlike ordinary cases where we apply empirical concepts, however, it is not obvious that we should expect this evidential basis to come in greater or lesser degrees. For example, suppose that strong representationalism is true. In this case, the phenomenal character of an experience would be an abstract property complex that the experience represents. Can an abstract property complex be present to greater or lesser degrees? It is hard to even make sense of this suggestion. First, a property complex is an abstract object, so the degree to which it is present cannot vary between situations. Second, abstract objects are plausibly fundamental in a sense that precludes being present to a certain degree at all. Consequently, according to at least one theory of phenomenal character, our evidential basis for applying our Consciousness concept will not come in greater or lesser degrees. Most importantly, this could be true even if borderline cases of consciousness are, in fact, possible. Because the sharpness of our evidential basis for applying our Consciousness concept does not imply that consciousness itself is sharp, our inability to conceive of situations where we recognize a borderline case of consciousness is not a good reason to conclude that borderline cases of consciousness are metaphysically impossible. Consequently, if the hypothesis presented above is accepted, we should reject premise 3 of the vagueness argument against physicalism.
In this post, I have provided two hypotheses that aim to explain when and why we are able to conceive of certain situations being actual. I have argued that, if these hypotheses are accepted, we have good reasons for rejecting key premises that occur in the zombie and vagueness arguments against physicalism. Whether these hypotheses are ultimately endorsed will depend upon wider considerations relating to modal epistemic theorizing, but they strike me as plausible and explanatorily powerful. For this reason, I believe physicalists have firm grounds for rejecting the zombie and vagueness arguments against physicalism.
Addendum:
According to the second explanatory hypotheses presented above, when we competently grasp a vague empirical concept, we should normally be able to conceive of situations in which our evidence for applying that concept is present in greater or lesser degrees. According to many non-representationalist theories of consciousness defended by physicalists, our Phenomenal Character concept is vague. Consequently, this hypothesis implies that we should be able to conceive of situations where phenomenal character is present to greater or lesser degrees. Tye, Antony, and others have persuasively argued that this is not the case. Furthermore, unlike the case of consciousness, a sharp evidential basis for applying our Phenomenal Character concept is not compatible with its being vague. This is because, if we apply our Phenomenal Character concept on the basis of our direct acquaintance with phenomenal character itself and phenomenal character admits of borderline cases, we should be able to imagine situations where the relevant evidence is present to greater or lesser degrees. Therefore, if we endorse the hypothesis under consideration, we should conclude that phenomenal character is sharp and reject any physicalist proposal that suggests otherwise (this would include the views proposed by David Papineau and Ned Block among others).
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