Wednesday, September 27, 2006

What's New in Quantum Gravity

I read Lee Smolin’s new book, The Trouble with Physics. I’m not going to review it in detail (here is a good review;UPDATE 3_Oct. see also review up at Cosmic Variance), but will instead focus below on the section where he discusses some newer approaches to quantum gravity.

The largest part of the book diagnoses the reasons for the slow progress in solving the big outstanding problems in theoretical physics. (Some of my earlier posts on this topic and referencing Smolin are here, here and here). The focus is on his perception of the shortcomings of string theory and also on the “sociological” issues which have led to string theory’s dominance of the field. He also makes suggestions for improving the situation so the next generation of physicists can make better progress.

While the focus is on string theory, it is also clear that the Loop Quantum Gravity program (with which Smolin has been most identified) has also made only slow progress – if it had been more successful, this book would not exist.

While these parts of the story left me a bit depressed, I still recommend the book for those interested in the topic: Smolin is a great writer and is well positioned to speak on the issues. (The book is frequently being reviewed in conjunction with Peter Woit’s new book, which I plan to read; in the meantime I continue to follow Woit’s interesting blog.)

Amidst the gloominess, Smolin does make some optimistic comments about the development of alternative background-independent (BI) approaches to quantum gravity (Chapter 15). In BI approaches, he says, one does not “start with space, or anything moving in space.” Instead, one starts with an abstract quantum mechanical structure, then looks for spacetime to emerge at larger scales. Early attempts proceeded by quantizing Einstein’s spacetime geometry directly (in the spirit of quantizing the classical electromagnetic field). These didn’t work, with the main problem being the generation of infinities in the expressions. A more sophisticated model, that of loop quantum gravity, has created finite outcomes, but (if my understanding is right) it works by encoding the spacetime of relativity directly into a quantum geometry. The complexity which comes from including the quantum states of all the geometric degrees of freedom in the model has made it difficult to get the dynamical four-dimensional spacetime back out again. [UPDATE (29 Sept.): See a few additional notes on LQG below in the comments.]

So, now, the idea is to take the BI approach even deeper and construct a quantum “pre-spacetime” theory. Looking at these approaches, Smolin also concludes they must include causality as a fundamental feature. In relativity, the light-cones implement a causal structure: you can tell which events precede or succeed others from a given reference frame. While we usually might think in terms of spacetime imposing the causal structure, Smolin says you could turn this around and say that causality determines the spacetime geometry. In this spirit, many researchers in quantum gravity now think causality is fundamental, and must feature in the construction of a pre-spacetime theory.

Here is Smolin’s summary of his meta-thoughts on quantum gravity theories:

“The most successful approaches to quantum gravity to date combine these three basic ideas: that space is emergent, that the more fundamental description is discrete, and that this description involves causality in a fundamental way. (emphasis original)”

Note he doesn’t say time is emergent. If causality is fundamental, then some notion of time is fundamental. However, it wouldn’t be a global time dimension we’re talking about: time would be localized at the level of the building blocks.

The chapter includes a survey of a number of approaches, however, the two newer ideas which Smolin seemed most enthusiastic about were a model called Causal Dynamic Triangulations and a new take on Quantum Causal Histories which utilizes an idea from quantum information processing to make spacetime emerge from a pre-spacetime reality. I will follow up with an attempt to look a bit more closely at these models.

Wednesday, September 13, 2006

The Russellian Stance & Concluding Thoughts

(This is the final post in a series, for background see the previous posts.) There is a well-known philosophical argument regarding the limits of our knowledge of physical objects and the relevance of this for the problem of consciousness. This argument is most prominently associated with Bertrand Russell, although there are connections with the work of many other philosophers. The argument has two parts. The first part asserts that we only know physical objects by their dispositional, relational, or extrinsic nature. Our knowledge of physical phenomena leaves untouched their categorical, non-relational, or intrinsic nature. In his book, Daniel Stoljar refers to this as the categorical argument. The second part of the argument draws a connection between the (hidden) intrinsic aspect of physical phenomena and the seemingly intrinsic quality of the phenomenal properties of first-person experience. (For other discussions online, see the “Type-F monism” section of this Chalmers paper; an earlier treatment of the view -- “o-physicalism” – in this Stoljar paper; and, for a more extended treatment of related philosophical theories, see this SEP article by Leopold Stubenberg. Also, Chapter 2 of Gregg Rosenberg’s book employs a creative argument in this vein).

Stoljar briefly gives his own account of the categorical argument. He divides physical truths into three exhaustive categories: spatio-temporal truths, truths about secondary qualities, and truths about primary qualities (note that properties from our physical theories like mass and charge would fall into this grouping). He discusses ways these individually or in combination can be seen to fail to tell the whole story about physical objects and specifically only tell a dispositional story about physical things. While some of the details could be contentious, the categorical argument's conclusion appears plausible.

There are two ways to take this argument further. For the first, note how the terrain here overlaps the discussion of the “structure and dynamics” objection which was the topic of my earlier post (although in Stoljar’s book, it comes in a later chapter). One could argue that the categorical argument leads directly to this thesis (Interpretation 1):

1. All non-experiential truths are (and will always be) dispositional.
2. Experience concerns categorical truths.
3. Therefore, non-experiential truths cannot entail experiential truths.

Stoljar’s epistemic thesis would be false – there cannot be non-experiential truths of which we are ignorant which are relevant to experience.

I should note that for a composite system (like a human), it can be said that our experience depends on both dispositional and categorical truths rather than exclusively categorical truths; but the above argument would still hold.

Stoljar argues differently. He wants to extend the categorical argument into a "categorical ignorance hypothesis" akin to, but more specific than, his own view. The key to this is an assertion that non-experiential truths could encompass both dispositional and categorical physical truths. Just because we don’t know of any examples of non-experiential categorical truths doesn’t mean there aren’t any. Note that Stoljar wouldn’t think it relevant even if our ignorance of this latter putative type of truth was chronic. Given this assumption, we have Interpretation 2:

1. Known non-experiential truths are dispositional
2. We are ignorant of categorical truths
3. Categorical truths may be both non-experiential and relevant to experience.

The resulting “Russellian version” of Stoljar’s epistemic view does dissolve CA and KA, and thus solves the logical problem of consciousness. (Note that Stoljar does not positively recommend this version of the Russellian view, rather he is advancing the more general version of the epistemic argument and is using the Russellian version to show an example of how the more general version could be fleshed out).


Stoljar’s thesis is difficult to defeat, because it is often easier to sow doubt about the adequacy of our knowledge than it is to defend positive assertions (like the statement above saying “all non-experiential truths are dispositional”).

In responding to Stoljar, I think the best strategy is to look more closely at how the paired terms non-experiential/experiential, dispositional/categorical, and objective/subjective are used and how they relate to each other. To do this, I will employ what I intend to be a minimal description of the world which must obtain to make sense of how these terms apply to it.

I start with the objective/subjective pair, because I think it is easiest to agree upon what these terms mean. In my last post I argued that objective and subjective truths are exhaustive of all truths. In essence, I’m saying that this is a dichotomy which describes our world. This doesn’t mean the world is described by a fundamental dualism like that of Descartes: both kinds of truth could supervene on an underlying common ground (neutral monism). On the other hand, a pluralism is implied, of course: The world contains individuated natural systems, and one type of truth obtains when a system participates in an event (subjective truth), and another type of truth obtains when such event is indirectly encountered (objective truth). There are as many “subjective” points-of-view as there are natural systems.

If you grant me this much, then the most natural interpretation of experiential truths is to identify them with subjective truths, while non-experiential truths are identified with objective truths. As discussed in the last post, such an interpretation defeats Stoljar’s thesis.

With regard to dispositional versus categorical truths, the terms are a bit more obscure, but the analysis leads to a similar result. A system’s indirect non-participatory knowledge of truths about other systems will only be of dispositional truths. We can only know them by how they interact with further entities. Another way to see this is to use the synonyms “relational” or “extrinsic” as alternative pointers to the same concept. The most straightforward interpretation would likewise identify non-dispositional (i.e. categorical) truths with those known through direct participation. The idea of a "non-experiential categorical truth" appealed to by Stoljar above has no room to fit into this picture and is not a well motivated concept.

Concluding Thoughts

Stoljar’s book was valuable to me not only for his interesting thesis, but for how useful it was as a touchstone for reflecting on the debates in (analytic) philosophy of mind which I’ve been reading about for around 15 years now. Within this philosophical domain, I have been most persuaded by the views I’ve discussed in these posts: a version of the Russellian stance, bolstered by arguments primarily associated with Chalmers and Nagel.

But, of course, consensus is far off. I haven’t discussed the parts of Stoljar’s book where he argues against competing physicalist perspectives on the problem of consciousness (arguments one agrees with always generate less scrutiny!). But many philosophers still subscribe to these arguments.

It seems that a significant move toward consensus will probably require input from outside philosophy of mind. Even in my “minimal description of the world” outlined above I was essentially “cheating” by bringing in ontological assertions about the existence of natural systems and how they relate to each other. But I think some sort of “triangulation” is necessary to make progress. Consciousness needs to fit into an improved metaphysical portrait. As an example, this is what appealed to me about Gregg Rosenberg’s thesis (see posts here), which combined insights from an analysis of causation and the composition of natural individuals to find the right “place for consciousness” in the world. It is also the reason I continue to be very interested in the philosophical interpretation of foundations of physics.

It seems right that a deeper understanding of consciousness will go hand in hand with a deeper understanding of nature.

Friday, September 08, 2006

The Objection from Objectivity

(This is the second post in a series; please see the previous post for background). The second objection considered by Daniel Stoljar in Chapter 8 of his book employs the idea of objectivity as the “Factor X” which characterizes non-experiential truths. Stoljar paraphrases this objection, which relates to an argument prominently associated with Thomas Nagel, as saying that non-experiential truths are and will always be objective (third-person) truths, and the problem of consciousness will always re-emerge since experiential truths are subjective (first-person) truths. I should declare my antecedent bias here that I have always found this argument to be extremely persuasive myself. If we invoke Nagel’s example of the bat: once I know all the objective truths about a bat, what it is like to be the bat is a further truth.

Now the way this objection is framed (that is, non-experiential truths are those known via the objective point of view and experiential truths are defined as those known through the subjective point of view) poses a challenge for Stoljar. He cannot argue against the following statement:

1. Even if you were to know all the objective (non-experiential) truths you would still not thereby know the subjective (experiential) truths.

Stoljar notes that this statement is analytic and flows from the terms of the objection. So he will try to form a reply by arguing that the following seemingly natural inference from statement #1 actually does not follow:

2. Even if you were to know all the objective facts, there will still appear to you to be an element of contingency in the relation between the objective and subjective facts.

It is this appearance of contingency or lack of entailment that is the basis for a third statement:

3. CA and KA will continue to be forceful no matter how many objective facts we learn.

Stoljar’s wants reject the inference from #1 to #2. His first strategy for doing this is to set up in parallel a counterexample thus:

4. John is in pain

This, he explains, is clearly a subjective truth. Then, since #4 is subjective then it negation must be as well:

5. John is not in pain

Next we have:

6. If John is a number, then he is not in pain.

Note this is a necessary truth. And since the antecedent of #6:

7. John is a number

is an objective truth, then we have an example where an objective truth entails a subjective truth. Putting this all in a form which mimics (1) and (2) above, we have:

8. Even if I were to know that John is a number, I would not thereby know that he is not in pain.

9. Even if I were to know that John is a number, there would still appear to me to be an element of contingency in the relation between John’s being a number and his not being in pain.

The inference from #8 to #9 is invalid, since #8 is true and #9 is false. Since this parallels #1 and #2, Stoljar says that inference is also false. When I read this the first time, I found the form of the counterexample fine, but its content was so different than #1 and #2 that I didn’t feel its force. I should also note he says we could use the slightly less strange “If John does not exist, then he is not in pain” as a substitute statement #6. But Stoljar senses the reader will need more help to flesh this all out.

So, continuing the discussion, Stoljar says he wants to distinguish the idea that the objective facts entail the subjective facts from the idea that understanding all the objective facts entails an understanding of the subjective ones. He introduces the idea that {if A then B} could be necessary but not synthesizable. Here’s his definition of synthesizable: “{If A then B} is synthesizable if and only if in every possible world in which S understands A, then S understands (or is in a position to understand) B – for short, if and only if understanding A entails understanding B.” Stoljar says the statement #1 above shows that subjective facts are not synthesizable from objective facts, but does not entail statement #2 – the relation could still be necessary and could appear to us as necessary, given future lifting of the veil of ignorance. To show that something could be not synthesizable but still appear necessary, Stoljar offers further examples such as this: if x is colored, then x is extended. Understanding about x being colored doesn’t entail understanding about what it is for it to be extended, but we can see that the conditional statement indeed describes a necessary relation given our possession of all the relevant facts. Again, in reading this, I found the content of Stoljar’s example to be different enough from that of #1 and #2 that I began to contemplate what could be faulty about the parallel.

For me, despite the examples, it was difficult to see that that one could ever concede the non-synthesizability of the subjective facts from the objective facts but come to find them appearing nonetheless entailed by them. But – and here again that is what makes the epistemic view slippery – Stoljar might say my feeling on this is just due to my ignorance! Can I flesh out a specific disanalogy in Stoljar’s examples which makes them lose force? I’ll give it a try.

The difference is that in the original case the two types of truths (objective and subjective) are presumed by definition to be exhaustive of all truths. In the cases of Stoljar’s examples, the additional facts which led us to see that {if A then B} could be necessary even when not synthesizable invoked other types of facts beyond the types represented by A and B. In the original counterexample #8 and #9 I need to know about abstract and concrete objects and what kind of objects can have pain. These facts were the “new” facts beyond just knowing everything there was to know about the antecedent “numberish” facts. But in the original objection, there are no new types of facts beyond the objective facts that could help me erase the appearance of contingency, because objective and subjective facts exhaust all facts.

I would note that I am not criticizing the distinction between a relation of necessity and one of synthesizability per se. Stoljar has used this effectively elsewhere (as in this paper). I just think it doesn’t do the job in this particular case.

Now I’d be grateful if anyone finds fault with my analysis here. For now, though, the reply Stoljar offers in this section was a rare passage in a clearly argued book which I found unconvincing.


The somewhat complicated reply to the objection from objectivity was unconvincing to me. I think there is still room to believe that the evident dichotomy of all truths into subjective and objective truths remains a key obstacle to dissolving the logical problem of consciousness in favor of a materialist stance.

Wednesday, September 06, 2006

The Objection from Structure and Dynamics

This is the first in a series of three posts which take a closer look at certain sections of Daniel Stoljar’s book Ignorance and Imagination: The Epistemic Origin of the Problem of Consciousness. In his Chapter 8 he discusses and replies to 2 possible objections to his thesis, and these will be the focus of this post and the next. In another follow-up post I want to discuss his treatment of the Russellian (“neutral monist”) stance.


To review, Stoljar usefully frames the logical problem of consciousness as follows. We have reason to endorse a triad of inconsistent statements:

1. There are experiential truths.
2. If there are experiential truths, every experiential truth is entailed by some non-experiential truth.
3. If there are experiential truths, NOT every experiential truth is entailed by some non-experiential truth.

Briefly and roughly, experiential truths are events of phenomenal and subjective character for which it is like something to undergo them. Stoljar (rightly in my opinion) takes #1 above to be obvious and does not spend much time on it. We have reason to believe #2 above given the track record of explaining manifest or folk truths in terms of scientific descriptions (which, to-date, all seem to coherently supervene on ground-level truths of physical theory). We might reasonably expect a correct treatment of experience in terms of non-experiential scientific truths to follow in due course. Someone holding to the philosophical position of physicalism (or materialism, to use the older term) accepts #2 and rejects #3. As an aside, I think Stoljar’s choice of using the term “non-experiential” rather than “physical” where possible is helpful, since it is always possible to get distracted in defining “physical” – although there are times where you need to revisit this.

We have reason to believe #3 above due to a number of related arguments, especially those known as the conceivability and knowledge arguments (“CA” and “KA”). For the purposes of my posts, I will presume familiarity with these; for a quick reference on these as well for the objection to Stoljar considered below, see David Chalmer’s paper Consciousness and its Place in Nature (this is also the paper which has his useful taxonomy of positions on the problem – Stoljar’s would be a form of “Type-C” materialism); for a more recent extended treatment of CA from Chalmers, see this paper. Dualists and other critics of physicalism accept #3 and reject #2.

In his book, Stoljar considers and rejects criticisms of CA/KA in the literature with the exception of his own, which he argues is successful. His (disarmingly modest-seeming) stance is an epistemic one, arguing CA/KA fail because we are ignorant of a type of “experience-relevant non-experiential truth”. It is only this ignorance which gives CA and KA their force, and if all the facts were known, we would all reject #3 and accept #2.

In his Chapter 8, Stoljar considers some possible objections and offers replies. He gives a general form of an objection (a master argument) as follows (some paraphrasing): we know we’re ignorant of many things, but we know enough to know that any relevant non-experiential truth is going to be characterized by some Factor X. The problem we have now which leads to the reasonableness of #3 will always re-emerge when we consider these new truths as a result.

Now, let me digress and admit that in similar discussions in the past I might have already given into a temptation to stop and protest: “how can it be that experiential truths can ever be entailed by (or “emerge” from) non-experiential truths? Isn’t the idea of “experience-relevant non-experiential truth incoherent?” Similar sentiments abounded in the Galen Strawson paper I praised and linked to a short while ago (btw, Stoljar has recently posted a response to the Strawson paper). But, in terms of Stoljar’s explicitly epistemic argument, one is not entitled to this response at this point. If the view is correct, then the notion only seems incoherent because of my ignorance. So, we must soldier on and put more flesh on what it is about non-experiential truths (Factor X) which has and will continue to make them incapable of appearing to entail experiential truths.

The Objection from “Structure and Dynamics”

The first candidate for Factor X Stoljar considers, stemming from Chalmers, invokes the idea that physical descriptions invariably characterize phenomena in terms of “structure and dynamics” (sometimes “structure and function” – see the first paper linked to above as well as Chapter 3, section 2 of The Conscious Mind). For any structural or functional explanation of consciousness, we’ll always be able to further ask why such a structure or function is accompanied by experience. Experience will not be entailed by the structural/functional explanation.

Stoljar, reading and interpreting Chalmers, says structure involves either a spatiotemporal relation or the property of playing a certain causal role – in other words what philosophers might also call dispositional or relational properties. Dynamics refers to how the system changes its dispositional or relational properties through time. So, instead of structure and dynamics, one could say “relations and dispositions”. Yet another way to put it is to say that we are referring to extrinsic properties. It is then asserted that these cannot be said to entail intrinsic properties, and experiential truths concern intrinsic properties. In replying to Chalmers, Stoljar refers to structure and dynamics, but says his reply would still hold force if we utilize these other terms.

Stoljar’s first reply to the objection is to say that while we can categorize physical descriptions in term of non-experiential structure and dynamics there also are experiential structure and dynamics. The experiential field has varying degrees of intensity, unity and dispositional qualities (such as a pain getting worse if I move a certain way). So, Stoljar asserts that if one knew everything about structure and dynamics, one would also know about some experiential structure and dynamics. Now, Chalmers might say that knowing about physical structure and dynamics will not help with the distinctively experiential variety of these. But why not? I think Stoljar’s response is effective up to this point.

Chalmers or his stand-in might say next, however, that experiential truths necessarily include another type of truth untouched by structure and dynamics. If so, Stoljar needs to know what this is so as not to beg the original question. A candidate might be intrinsic (non-relational, non-dispositional) properties. Stoljar next replies to this suggestion. He says while it seems that experiences have intrinsic properties (such as the blueness of an expanse of blue sky), it is really the representation of the sky which has the intrinsic property not the experience itself. I thought this was a weak response. It invokes a distinct and controversial argument regarding the representational nature of experience which is orthogonal to the debate at hand.

But Stoljar then offers a second reply to this revised version of the objection (to recapitulate: the idea is that experiential truths sometimes involve intrinsic properties, non-experiential truths never do, and you can’t get derive intrinsic truths from extrinsic ones). He offers examples such as the following: being a husband is an extrinsic (relational) property of Jack Spratt; and being a wife is an extrinsic property of Jack's wife; but being married is an intrinsic property of them as a pair. So the intrinsic fact of the whole did indeed derive from the relational facts of the parts. But, one might respond that this part/whole discussion misses the point when it is argued that it is the extrinsic description of a single thing that will fail to entail the intrinsic nature of that same thing. But, Stoljar seems comfortable to leave things here, thinking one can always assert a relational parts->intrinsic whole for any complex object, and since our starting point is the experience of a complex human individual, this reply has some force. The objector has just one defense left: he could assert that the world has an intrinsic nature at its most basic indivisible level. At this point, the dialectic stops, since the idea that all physical reality has an intrinsic nature relates to the Russellian view that Stoljar treats separately (as will I).


I think Stoljar’s reply to the objection from structure and dynamics is successful up to a point. He pushes the objector toward arguing for an intrinsic nature at the basic level of the world, rather than for a composite individual such as a human per se.