Thomas Nagel and the Principle of Sufficient Reason (or, on unprincipled natural teleology)

[Thomas Nagel's Mind & Cosmos has generated a number of very critical reviews, including this blistering one by Leiter/Weisberg, and another, wide-ranging one by Eliott Sober (thanked for his refereeing at OUP by Nagel in the preface to M&C!) as well as gratitude from the creationists (recall Mohan's discussion). Mohan has blogged about some of Nagel's moves before,  but the rather long post -- apologies -- below engages with issues thus far unexplored in these discussions.--ES.]

Analytical philosophy has made great progress over the last century. But its original, necessary biases did some harm, too. In particular, detailed working knowledge of the history of philosophy and metaphysics was banished for several generations. While metaphysics is thriving again, we still lack (despite the brilliance of David Lewis' modular approach) complete systems of thought that can rival in depth and interlocking breadth the past masters (say, Suarez, Leibniz, etc.). The damage has also been more narrow. For example, one of the most obvious so-called ‘Kuhn Losses’ is our relative ignorance of the nature and implications of the Principle of Sufficient Reason (PSR). This is no surprise because analytical philosophy was founded in the act of rejecting PSR. Our forefathers’ attempt to balance between common sense and the truths of science meant -- as science and the PSR parted ways -- the willing submission to brute, ultimate facts (recall this post).  

In Mind & Cosmos, Thomas Nagel happily embraces “a form of the principle of sufficient reason” (17) in support of his "common sense" (5, 7, etc.) and against the recent “orthodox scientific consensus.” (10; 5) Rather than accepting this "ideological consensus," (128) Nagel insists -- regularly using language reminiscent of the great Feyerabend --  that "almost everyone in our secular culture has been browbeaten into regarding the reductive research program as sacrosanct." (7) While Nagel insists that the champions of scientific enlightenment are bullies, he treats the "defenders of intelligent design" with "gratitude" (Plantinga returns the gratitude),  even though Nagel clearly recognizes that once one embraces one's inner sensus divinitatis one is also compelled in one's judgments. (12) 

A classic statement of the PSR is Spinoza's "For each thing there must be assigned a cause, or reason, both for its existence and for its nonexistence." (Ethics 1p11d2) That is to say, any PSR worth having imposes significant explanatory demands (especially of non-arbitrariness) on any philosophical system in which it is deployed. Below the fold I critically discuss Nagel's way of combining the PSR and his attempted revisionary science, but here I just register the marvelousness of Nagel's deployment of the PSR as an instrument in the service of common sense! (cf. 91-2) This is certainly an original move in the history of metaphysics--one that, in a single, magical stroke overturns Lovejoy's long narrative.

Above I claimed that natural science and the PSR had parted ways some time before the revolt of analytical philosophy against the PSR, so that given the choice between the PSR and science, Russell (et al.) opted to accept "submission to fact" rather than Idealist flights of fancy (recall my post). It should come as no surprise, then, that Nagel rejects "inexplicable...brute" facts (45 [discussing consciousness in materialist terms]) and understands himself as an "objective idealist" (such as "Schelling and Hegel" (17); Nagel also reads the "logical positivists" and "sense data" theorists, more generally, as "idealists"). Nagel, however, thinks that science itself presupposes a (mild) version of the PSR. In particular, "the assumption of intelligibility has led to extraordinary discoveries...one cannot really understand the scientific world view unless one assumes the intelligibility of the world, as described by the laws that science has uncovered, is itself part of the deepest explanation why things are as they are." (16-17) Nagel is entitled to the identification of (scientific) law with intelligibility (see also 61), but (a) explanations that satisfy the PSR are generally more demanding than mere intelligibility (recall the quote from Spinoza above), and (b) Nagel does not accept this standard of intelligibility as sufficient. (Moreover, in science severe empirical testing is a more important touchstone than the PSR or intelligibility.) In a future post I explore Nagel's views on the intelligible (and to what degree his purported common sense is un-common).

Nagel wants to reintroduce natural teleology into modern science. Nagel falsely thinks that teleology was "banished from the scene at the birth of science." (66) This would have come as a surprise to Boyle, Newton, Leibniz all of whom embraced teleology in some non-trivial sense (and in different ways). As an aside, Nagel also mistakenly claims that the "scientific revolution" was itself made possible by an embrace of the primary-secondary distinction that some how left "mind" out of the emerging picture ((35) with nods to "Galileo and Descartes"). This (recall) is a popular myth (often recycled by Heideggerians who have read Husserl), but if true it would leave the greatest scientific revolutionary, Isaac Newton, who at one point in the lead up to the Principia even defines bodies in terms of their ability to produce effects on minds, outside his own revolution. There are interesting historical-philosophical question about when physics and materialism came to be thought co-extensive, and to what degree teleology was ever fully banished from science (this involving tough issues about variational principles). 

Here I focus on Nagel's preferred natural teleological approach, which draws heavily on Hawthorne and Nolan (67, 93); in Nagel's hands teleology requires:

First, that the nonteleological and timeless laws of physics--those governing the ultimate elements of the physical universe, whatever they are--are not fully deterministic. Given the physical state of the universe at any moment, the laws of physics would have to leave open a range of alternative successor states, presumably with a probability distribution over them.

Second, among those possible futures there will be some that are more eligible than others as possible steps on the way to the formation of more complex systems, and ultimately of the kinds of replicating systems characteristic of life. The existence of teleology requires that successor states in this subset have a significantly higher probability than is entailed by the laws of physics alone--simply because they are on the path toward a certain outcome. Teleological laws would assign higher probability to steps on paths in state space that have a higher "velocity" toward certain outcomes. [Note to Hawthorne and Nolan] They would be laws of the self-organization of matter, essentially--or of whatever is more basic than matter. (92-3)

Hawthorne and Nolan's account of "end-velocity laws" is ingenious. Let's also grant them the use of the conceptual apparatus of Hilbert spaces (they are mostly discussing laws governing particles.) Yet, unlike Nagel they quite openly flirt with the possibility that the fundamental physical laws (of the "world-system") are themselves teleological (in their sense). At the most general level, their account is not entirely satisfactory because (as they recognize) it denies time-reversibility (which is such a striking feature of  many (but certainly not all) fundamental physical laws), but in what follows nothing will turn on this.

Hawthorne and Nolan make no appeal to the PSR; in fact, they explicitly flirt with denying “ex nihilo, nihil fit.” So, their natural teleology self-consciously opens the door to brute facts or to God's agency. Given that Nagel explicitly wishes to steer a path in between brute facts and God's agency, his adoption of the Hawthorne and Nolan framework is opportunistic. Moreover, while I am not claiming that the PSR and probabilistic laws must always be in conflict with each other, it is striking that Nagel must embrace as a brute fact that some futures are "more eligible" than others. (Once granted, teleology takes over.) From the point of view of any PSR worth having this is all just extremely arbitrary.

Worse, Nagel has an entirely arbitrary distinction between the fundamental non-teleological and the required- for-consciousness-non-fundamental teleological laws. We are not offered an account of the relationship between the two kinds of laws, nor an explanation why the non-fundamental-teleological laws occur in Hilbert spaces (and how the entities inhabiting these connect to consciousness and points of view).

Moreover,  these-non-fundamental teleological laws are laws of self-organizing matter (see also (59)), but somehow independent of the fundamental non-teleological laws. Even if that were sufficient reason for that, it is not at all obvious how we move from self-organizing matter to conscious matter. (cf. Nagel (52)). Spinoza and Leibniz certainly saw the attraction of self-organizing matter, but they recognized that for 'mindedness' additional resources are required. For an account that regularly appeals to intelligibility (17, 26-29, 52), Nagel's leaves a lot of unexplained gaps. But from the point of view of the PSR this is all just very ad hoc, not to say unprincipled. (Hence, Nagel offers them "without positive conviction" (124)).

In conclusion, unlike some of my Darwinian friends I think we should keep philosophy a welcoming place to folk that want to explore possible future sciences in opposition to present science. (This is why I could see the positive in Glymour's ranting way back when.) But leaving aside Nagel's systematic underestimation of the resources available to the Darwinian program, Nagel's approach is a philosophical mess. He appeals to intelligibility and the PSR when it suits him, but without applying the PSR to his own commitments. 

I am all for philosophical messiness and opportunism if there were ways to transform Nagel's aspirations into fruitful theory-mediated measurements available to future scientific researchers, but Nagel is not really in the business of offering conceptual guidance to a future science. Rather, after a life-time spent at the pinnacle of professional philosophy, Nagel's soul longs for what he calls "reassuring" explanations (25-27; about which more in the future). Nagel closes his book with "the human will to believe is inexhaustible," (128). Quoting Psalm 139, Alvin Plantinga is surely right to insist that "if" Nagel "followed his own arguments wherever they lead," Nagel would end up with (Christianized) theism. Some such religion is a useful adaptation for souls longing for reassurance.