Observer Dependent Physicalism: A New Argument for Reductive Physicalism and for Scientific Realism

Abstract

Reductive physicalism is a minority view in contemporary philosophy as well as in science, and therefore arguments for endorsing it often amount to arguments against the alternative views, in particular so-called non-reductive physicalism. In this paper we put forward a new argument for reductive physicalism, according to which it is the best account of the empirical data that we have. In particular, we show that: (a) a reductive physicalist theory of the mind forms an essential part of the very argument for the existence of an external world: if one rejects psychophysical identity physicalism, one’s case for believing in the existence of an external world is substantially weakened; and (b) the success of a physical account of the mind is the most important criterion for the success of (future) physics; and (c) reductive physicalism is the only approach in which relying on science in order to account for the external world makes sense; and finally, (d) within reductive physicalism, one can provide a new criterion for theory preference in science, and a new argument for scientific realism. In our physicalist view the experience forms the initial data, which is then reduced to fundamental physics.

Appendices

Appendices

1.1 Appendix A: Why So-Called Non-reductive Physicalism Is a Form of Dualism

Consider three token microstates (call them A, B, and C) such that A and B are of the same biological type (call this type M) but C isn’t. This is a fact about the universe, and in particular, a fact about A, B, and C and M. A, B, and C can be microstates of the universe at three different moments, or three sub-microstates of the universe, i.e. the microstate as it describes three separable subsystems of the universe. What makes A and B, but not C cases of the same type M? In particular, since Flat Physicalism is committed to the assumption that everything is physical, it must provide an answer to the question: what physical facts make it the case that the particular tokens A and B belong to the same biological type M while C doesn’t? Two options come to mind.

• (I) One obvious case in which Flat Physicalism can account for this fact is when the microstates A and B share an aspect, which is the biological fact of being M; whereas C doesn’t have this aspect. If an observer happens to be sensitive to that biological aspect, then this observer will not be able to distinguish between A and B, or to tell whether the microstate that actually takes place is A or B. (If the observer is also sensitive to other aspects of A and B, in which they differ, then the observer will be able to tell which of them is the case.) But importantly, A and B share this biological aspect whether or not there is an observer that is sensitive to it; Whether or not two microstates belong to the same biological kind depends on these microstates only, regardless of whether or not anyone observes them.

• (II) Suppose, however, that the following happens (and whether or not it happens in our universe is a question of fact). The microstates A and B are of the same biological type M, despite the fact that they do not share any aspect that is shared by them and only by them (and not by e.g., C which is not M), and that is relevant for the biological type M in the sense that it can – according to one’s theory – account for the fact that they are tokens of M; in this case we shall say that they are physically heterogeneous. Or, if they do share some aspect, then suppose (by hypothesis) that this aspect is not relevant for the biological kind in question (for otherwise we would be back to the previous case I).

This case is important, because it seems to be a case of so-called multiple realization of biological (and other special sciences) facts by physical facts. (Recall that above we said that the case of a set of microstates that share an aspect and thus forms a macrostate is – in our terminology – not one of multiple realization, despite the fact that these microstates, in general, differ from each other in other aspects. The reader may employ their favourite understanding of this term, mutatis mutandis.)

Flat Physicalism, which is a theory of reductive physicalism, can explain this case as follows.

Suppose that the total microstate of the universe consists of (at least) four parts (i.e. There are four subsystems of the universe, and the state space of the universe can be separated to parts that correspond to their degrees of freedom): the four sub-microstates are our microstates A, B, and C (we now focus on A and B) and a fourth part with its microstate D, such that whenever microstates A or B obtain, microstate D has the aspect Y. In this case, the extended microstates A + D and B + D share the aspect Y of D.

Importantly, the subsystem of the universe whose microstate is D is not a measuring device that measures properties of the systems whose microstates are A and B; aspect Y of D is not a measurement outcome; and the reason is that – by hypothesis – A and B do not share any physical aspect that could be reflected by the aspect Y of D. In other words, if a measurement did take place here, then this would be a case of (I), not of (II). Case (II) obtains when the fact that D has the aspect Y when either A or B obtain is a sort of factual correlation, due perhaps to some pre-established harmony or to pure coincidence.

If one, when observing A + D and B + D, perceives the shared aspect Y, but for some reason ignores the system whose microstate is D, one may get the impression that the fact that A and B are of the same biological kind M has no physical explanation. This may happen if, for example, D is the microstate of our own brain, so that when we observe A and B in our environment, we neglect to see ourselves as part of the extended microstate, and therefore the fact that A and B are of the same kind seems to us “molto mysterioso” (Fodor, 1974, 1997).

Notice that the system D here must be physical, in Flat Physicalism. In the case where D is a human observer, this calls for a reductive physicalist account of the mind, which is indeed a central tenet or commitment of Flat Physicalism.

• (III) Finally, take one step further: suppose that when microstate A takes place, D has the aspect Y; but when microstate B takes place, D has the aspect Z; so that A + D and B + D are physically heterogeneous; we don’t even have a shared aspect of extended microstates. (Or, if they share some physical fact, this fact is irrelevant for forming the type M, since otherwise we are back in case (I) above.) Which physical fact, then, makes it the case that A and B are of the same biological kind M? Here, there is no further shared physical aspect to fall onto; the multiple realization is genuine; and therefore the fact in virtue of which these two microstates belong to the same biological kind must be a non-physical fact. This is dualism, not physicalism. Furthermore: this is a case of token dualism, since the non-physical fact has to obtain in each and every token of the microstates that belong to the same biological kind. Importantly, (a) “dualism” is not a pejorative term, and may be true of the world; (b) if one objects to using the term “dualism” here, reserving it to some sort of psychophysical dualism, that is fine with us, we don’t insist on terminology.²⁷

If we know, say by a word from God, which physical aspects (kinds) fall under each high-level kind and multiply-realize it, then, given a physical microstate, we are able to deduce which high-level kind it falls under. But if we are not given a word from God, if multiple-realization is possible (and even if supervenience holds!) then there is no way that we can deduce from the physical making of the world, that is, from the microstate of the world, which aspects (kinds) fall under each high-level kind. If multiple realization is allowed, then there has to be a non-physical fact in play in order to fix the partition of the token microstates into the high-level special-sciences types. Fodor (1997), it seems, did not joke at all when he said that in non-reductive physicalism only God can fix these sets. Perhaps he didn’t mean God; perhaps he only meant that this is a brute fact; but his brute fact is over and above the complete physics of the world, and this is metaphysical dualism.

Notice that supervenience makes no difference here: if God already gives us the partition to special sciences sets, then we need supervenience in order to deduce to which high-level kind a given token microstate belongs; but if He doesn’t give us the partition, supervenience doesn’t help us do this.

A popular response to the above argument is that, despite first impression, the fact that A and B are M but C isn’t does have an explanation that can be deemed “physical”, namely: the states A and B (but not C) have (or realize or implement etc.) the same “functional role” or “causal role” or “computational role” within the appropriate physical context, despite being physically heterogeneous.

What precisely is the nature of this shared fact, namely, the function and the functional role? Consider one token of a function (or a causal process, or a computation), one particular “physical realization” (etc.) of that function. In terms of the fundamental physics (which is everything that there is, according to Flat Physicalism,) what we have here is a sequence of microstates, that is, we have here a segment of a micro-trajectory; and if A is one of the microstates along it, which occupies the functional role in which we are interested, we call that sequence SA. The second “physical realization” (etc.) of the same function is also a segment of a physical micro-trajectory; and if B is the microstate along it which occupies the functional role of interest, we call that sequence SB. In virtue of which fact do these two micro-trajectories segments SA and SB belong to the same set, that is: in virtue of which fact do SA and SB realize the same function, in which A and B may share the same functional role?

One option is that they share a physical aspect of that sequence, completely given in terms of the fundamental physical level; but then they would be a physical kind, not merely a functional kind, and we are back to case (I) above. If we are to have a genuine case of non-reductive physicalism, that is: of multiple realization of the functional kind by physical kinds, then the micro-trajectory segments have to be physically heterogeneous, that is, the fact shared by them has to be non-physical. And this is case (III) above, in which each token of the micro-trajectory segments has a non-physical aspect, so that we have here a case of token dualism. And so, all forms of so-called non-reductive physicalism are forms of dualism, not of physicalism.

As we already emphasized, if one objects to the term “dualism” we are happy to change terminology as long as we understand what it is about.

1.2 Appendix B: Identity Statements

In this appendix we briefly address the nature of identity statements, which are the core of type-type identity reductive physicalism. We follow Kripke’s (1980) approach in the analysis of identity statements in which the designators are rigid. In particular, we take it (following Kripke, 1980) that the designators in identity statements involving natural kinds (which for us are aspects of the world’s microstates) are rigid, but we don’t argue for this point here. To avoid confusion we wish to stress two points. (i) Our aim is to clarify the nature of such identity statements regardless of whether they are true or false, in particular regardless of whether one accepts or rejects psycho-physical identity physicalism. Famously, Kripke (1980) rejects psycho-physical identity theories on the basis of his so-called conceivability argument. As we will show below, our analysis of identity statements, which strictly follows Kripke’s (1980) analysis, is neutral with respect to this argument. In particular, we show that Kripke’s analysis of identity statements by itself, even if one accepts it, is no argument against reductive identity physicalism.²⁸ (ii) Our analysis appeals to the idea of reference following Kripke (1980). However, we stress that in a physicalist approach, there can be no reference in the sense of Kripke. In this approach one has to replace talk about reference with something else which is compatible with physicalism, in particular its non-relational understanding of the mind. We don’t address this last point here.

Physicalism is the idea that:

• (a) Everything is physical,

and in particular psychophysical identity theory is the idea that.

• (b) The data (concerning our mental experience, understood non-relationally, as in our explanatory schema) are physical.

Assuming that “pain” is a particular case of the data that appear in (b), and that “C-fibers firing” (following Kripke’s (1980) famous example) is a particular case of “the physical” in (b), the following statement is a special case of (b) as well as of (a):

• (c) Pain is C-fibers firing.²⁹

Following Kripke (1980) we understand (c) as an identity statement between rigid designators which, as such, if true, is necessarily true. Why is that so? Simply put, an identity statement tells us that whereas we thought – for whatever reason – that there are two things (or two kinds of things), there is only one thing (or kind of things), and so, saying that (in some possible world) the one exists and the other doesn’t amounts to saying that in that possible world that thing exists and doesn’t exist. (How does it come to be, or how is it even possible, that one thinks that there are two things while in fact there is only one, is a complex issue that we do not address here.)

(c) is not entailed by (a) nor by (b), because it may turn out that although everything including the mental is physical, pain is identical with some other kind of brain states or something else altogether. (This doesn’t make (c) contingent! As Kripke stresses, and we agree, (c) is a scientific conjecture, and if it is true, it is necessarily true.) Likewise, (a) or (b) are not entailed by (c) because it may be the case that (c) is true but other identity statements that are cases of (a) or (b) are false, or that (c) is false but other identity statements that are cases of (a) or (b) are true. Kripke thinks that the last option is the case. In fact, Kripke thinks that (c) is false even if one thinks of “C-fibers firing” as a placeholder for any physical (or biochemical, neurological, etc.) kind, and this is part of his conceivability argument supporting mind-body dualism (we don’t address this argument here).

Here is another psycho-physical identity statement, with the same structure and status as (c):

• (d) The sensation of heat is aspect H of the brain.

We mention (d) here since it plays a role in our analysis below, and it is important to see from the start its similarity with (c). “The sensation of heat” is the name (it is not a description) that Kripke gives to the sensation that is causally brought about by heat in the environment; like pain in (c), the sensation of heat is the name of a mental state. Similarly, “Aspect H of the brain” is the name that we give to the material feature of the world which, according to physicalism, is (identical with) the sensation of heat, in the same sense that C-fibers firing is (identical with) pain.

As part of his argument against physicalism, Kripke compares sentences like (c) with sentences like (e):

• (e) Heat is the motion of molecules.

Like (c), (e) is an identity statement concerning the references of rigid designators, and so if it is true it is necessarily true. Kripke thinks that (e) may be true if science is correct, unlike (c) which is false regardless of any scientific discoveries. (Again, this last point is the conclusion of Kripke’s conceivability argument with respect to which our present analysis is neutral, and we don’t address it here.)

In (e), the term “the motion of molecules” (according to science) refers to an aspect (or macrovariable) of the microstate of a system S in our environment. This aspect was pre-scientifically called ‘heat’: pre-scientifically it had a name, even though its nature was unknown, but now we know its nature (presumably, for the sake of the argument): it is the motion of molecules. And so (e) tells us that the nature of what we pre-scientifically called ‘heat’ is that it is the motion of molecules.³⁰

While pointing out that both (c) and (e) are identity statements expressed by rigid designators, Kripke analyzes the difference between these statements. (Indeed, the statement that is a more direct counterpart of (c) is (d); we discuss (d) below.) In particular, Kripke brought to our attention that (e) might be understood as referring implicitly to the causal relationship between two systems, for example: a system capable of having mental states (such as humans), call it U, and a system in its environment, call it S. The relation between S and U, which is important for the difference between (c) and (e), is this.

There is an aspect of the microstate of system S (in the environment of U), that in our pre-scientific theory of the world we call “heat” (but pre-scientifically we don’t know its nature). The aspect of S called “heat” casually³¹ brings about in U what we pre-scientifically call, due to its cause, “a sensation of heat” (an expression we use following Kripke). But note already here that the material feature of the world which, according to physicalism, is (identical with) the sensation of heat need not have anything to do with heat, namely, if (e) is true, with the motion of molecules).

Kripke made us aware that we sometimes use the word “heat” instead of the phrase “sensation of heat” to refer to the mental state of U, but this terminology is confusing and using it makes the reference of “heat” and the meaning of (e) unclear. Therefore in the present analysis we try to avoid this mistake.

And so:

• (f) “Heat”, which is the name of a feature of S (the nature of which was pre-scientifically unknown,) brings about in U a sensation, which we call “sensation of heat” (and is sometimes misleadingly called “heat” as well).

Science discovers that the aspect of S that brings about the sensation of heat in U, is the motion of molecules in S. And so:

• (g) The motion of molecules in S, which is an aspect of the microstate of S, brings about in U a sensation, which we call “sensation of heat” (and is sometimes misleadingly called “heat” as well).

According to (f) and (g), science discovered the deep structure of the feature of S called “heat”. By contrast, the analysis so far is silent as to the deep structure of the feature of U called “sensation of heat” (or, misleadingly, “heat”). Needless to say, understanding the deep structure of the “sensation of heat” is necessary in order to understand (g), namely, to understand how exactly the motion of molecules in S brings about the sensation of heat in U. According to Flat Physicalism, the deep structure of the feature of U called “sensation of heat” is given by the following identity statement:

• (h) The sensation of heat in U is an aspect of the microstate of U; call it aspect H of the brain of U.

Recall statement (d) from above:

(d) The sensation of heat is aspect H of the brain.

(h) and (d) are the same statement, phrased differently. We phrased (d) in this way in order for it to have the exact same structure as Kripke’s (c). (c) is the counterpart of (d), and the counterpart of (e) that would be best in order to understand features of (c) would be something like this:

• (i) “Stabbing is damaging bodily tissues,”

where stabbing in (i) is the cause of pain in (c), in the same way that heat in (e) is the cause of the sensation of heat in (d). The relation between statements (i) and (c) is the same as the relation between (e) and (d).

According to Flat Physicalism, the full story combines (e) and (d) as follows:

• (j) The motion of molecules in S, which is an aspect of the microstate of S (this aspect is pre-scientifically called “heat”), brings about the aspect H of the brain in U (this aspect is pre-scientifically called “sensation of heat” and sometimes “heat”).

Similarly, the full story that combines (e) and (d) is this:

• (k) Damaged bodily tissues, which is an aspect of the microstate of S (this aspect is pre-scientifically called “Stabbing”), brings about the aspect C-fibers firing of the brain in U (this aspect is pre-scientifically called “pain”).

Using the examples of statements (c) and (e) Kripke made two points.

One is that identity statements cannot be contingently true: if they are true, they are necessarily true. We accept this part of Kripke’s argument, and in this appendix we tried to clarify some ideas concerning this argument in light of these examples.

The other point that Kripke tried to make is that statement (c) is false, and therefore also statements (b) and (a) are false. This he tried to show using his conceivability argument: Since (c) is an identity statement between rigid designators, if it is true it is necessarily true. But – Kripke claims, on the basis of a strong and entrenched intuition – it is conceivable that in some possible world pain takes place without C-fibers firing taking place. Therefore (c) is not necessarily true, and therefore it is not true at all. We do not share Kripke’s intuition: we think it is not conceivable at all that (c) could take place without C-fibers firing taking place, no more that it is conceivable – given contemporary science – that heat could take place without molecular motion taking place. But arguing for this is beyond the aim of this appendix. It is sometimes thought that Kripke’s analysis of identity statements itself (regardless of intuitions about conceivability) undermines psycho-physical type-identity theories. As can be seen from our analysis, this is not the case.