Coding Information While Operators Fail to Commute. Drunken Risibility.

Suppose ∇ is a derivative operator on the manifold M. Then there is a (unique) smooth tensor field R^a_bcd on M such that for all smooth fields ξ^b,

R^a_bcd ξ^b = −2∇[_c∇_d] ξ^a —– (1)

Uniqueness is immediate since any two fields that satisfied this condition would agree in their action on all vectors ξ^b at all points. For existence, we introduce a field R^a_bcd and do so in such a way that it is clear that it satisfies the required condition. Let p be any point in M and let ξ’^b be any vector at p. We define R^a_bcd ξ’^b by considering any smooth field ξ^b on M that assumes the value ξ’^b at p and setting R^a_bcdξ’^b = −2∇[_c∇_d]ξ^a. It suffices to verify that the choice of the field ξ^b plays no role. For this it suffices to show that if η^b is a smooth field on M that vanishes at p, then necessarily ∇[_c∇_d] η^b vanishes at p as well. (For then we can apply this result, taking η^b to be the difference between any two candidates for ξ^b.)

The usual argument works. Let λ_a be any smooth field on M. Then we have,

0 = ∇[_c∇_d] (η^aλ_a) = ∇[_c η^a ∇_d] λ_a + η^a[_c∇_d] λ_a  + (∇[_c λ_|a|) (∇_d] η^a) + λ_a ∇[_c∇_d] η^a —– (2)

It is to be noted that in the third term of the final sum the vertical lines around the index indicate that it is not to be included in the anti-symmetrization. Now the first and third terms in that sum cancel each other. And the second vanishes at p. So we have 0= λ_a∇[_c ∇_d]η^a at p. But the field λ_a can be chosen so that it assumes any particular value at p. So ∇[_c ∇_d] η^a = 0 at p.

R^a_bcd is called the Riemann curvature tensor field (associated with ∇). It codes information about the degree to which the operators ∇_c and ∇_d fail to commute.

Transcendentally Realist Modality. Thought of the Day 78.1

Let us start at the beginning first! Though the fact is not mentioned in Genesis, the first thing God said on the first day of creation was ‘Let there be necessity’. And there was necessity. And God saw necessity, that it was good. And God divided necessity from contingency. And only then did He say ‘Let there be light’. Several days later, Adam and Eve were introducing names for the animals into their language, and during a break between the fish and the birds, introduced also into their language modal auxiliary verbs, or devices that would be translated into English using modal auxiliary verbs, and rules for their use, rules according to which it can be said of some things that they ‘could’ have been otherwise, and of other things that they ‘could not’. In so doing they were merely putting labels on a distinction that was no more their creation than were the fishes of the sea or the beasts of the field or the birds of the air.

And here is the rival view. The failure of Genesis to mention any command ‘Let there be necessity’ is to be explained simply by the fact that no such command was issued. We have no reason to suppose that the language in which God speaks to the angels contains modal auxiliary verbs or any equivalent device. Sometime after the Tower of Babel some tribes found that their purposes would be better served by introducing into their language certain modal auxiliary verbs, and fixing certain rules for their use. When we say that this is necessary while that is contingent, we are applying such rules, rules that are products of human, not divine intelligence.

This theological language would have been the natural way for seventeenth or eighteenth century philosophers, who nearly all were or professed to be theists or deists, to discuss the matter. For many today, such language cannot be literally accepted, and if it is only taken metaphorically, then at least better than those who speak figuratively and frame the question as that of whether the ‘origin’ of necessity lies outside us or within us. So let us drop the theological language, and try again.

Well, here the first view: Ultimately reality as it is in itself, independently of our attempts to conceptualize and comprehend it, contains both facts about what is, and superfacts about what not only is but had to have been. Our modal usages, for instance, the distinction between the simple indicative ‘is’ and the construction ‘had to have been’, simply reflect this fundamental distinction in the world, a distinction that is and from the beginning always was there, independently of us and our concerns.

And here is the second view: We have reasons, connected with our various purposes in life, to use certain words, including ‘would’ and ‘might’, in certain ways, and thereby to make certain distinctions. The distinction between those things in the world that would have been no matter what and those that might have failed to be if only is a projection of the distinctions made in our language. Our saying there were necessities there before us is a retroactive application to the pre-human world of a way of speaking invented and created by human beings in order to solve human problems.

Well, that’s the second try. With it even if one has gotten rid of theology, unfortunately one has not gotten rid of all metaphors. The key remaining metaphor is the optical one: reflection vs projection. Perhaps the attempt should be to get rid of all metaphors, and admit that the two views are not so much philosophical theses or doctrines as ‘metaphilosophical’ attitudes or orientations: a stance that finds the ‘reflection’ metaphor congenial, and the stance that finds the ‘projection’ metaphor congenial. So, lets try a third time to describe the distinction between the two outlooks in literal terms, avoiding optics as well as theology.

To begin with, both sides grant that there is a correspondence or parallelism between two items. On the one hand, there are facts about the contrast between what is necessary and what is contingent. On the other hand, there are facts about our usage of modal auxiliary verbs such as ‘would’ and ‘might’, and these include, for instance, the fact that we have no use for questions of the form ‘Would 29 still have been a prime number if such-and- such?’ but may have use for questions of the form ‘Would 29 still have been the number of years it takes for Saturn to orbit the sun if such-and-such?’ The difference between the two sides concerns the order of explanation of the relation between the two parallel ranges of facts.

And what is meant by that? Well, both sides grant that ‘29 is necessarily prime’, for instance, is a proper thing to say, but they differ in the explanation why it is a proper thing to say. Asked why, the first side will say that ultimately it is simply because 29 is necessarily prime. That makes the proposition that 29 is necessarily prime true, and since the sentence ‘29 is necessarily prime’ expresses that proposition, it is true also, and a proper thing to say. The second side will say instead that ‘29 is necessarily prime’ is a proper thing to say because there is a rule of our language according to which it is a proper thing to say. This formulation of the difference between the two sides gets rid of metaphor, though it does put an awful lot of weight on the perhaps fragile ‘why’ and ‘because’.

Note that the adherents of the second view need not deny that 29 is necessarily prime. On the contrary, having said that the sentence ‘29 is necessarily prime’ is, per rules of our language, a proper thing to say, they will go on to say it. Nor need the adherents of the first view deny that recognition of the propriety of saying ‘29 is necessarily prime’ is enshrined in a rule of our language. The adherents of the first view need not even deny that proximately, as individuals, we learn that ‘29 is necessarily prime’ is a proper thing to say by picking up the pertinent rule in the course of learning our language. But the adherents of the first view will maintain that the rule itself is only proper because collectively, as the creators of the language, we or our remote answers have, in setting up the rule, managed to achieve correspondence with a pre-existing fact, or rather, a pre-existing superfact, the superfact that 29 is necessarily prime. The difference between the two views is, in the order of explanation.

The adherents regarding labels for the two sides, or ‘metaphilosophical’ stances, rather than inventing new ones, will simply take two of the most overworked terms in the philosophical lexicon and give them one more job to do, calling the reflection view ‘realism’ about modality, and the projection view ‘pragmatism’. That at least will be easy to remember, since ‘realism’ and ‘reflection’ begin with the same first two letters, as do ‘pragmatism’ and ‘projection’. The realist/pragmatist distinction has bearing across a range of issues and problems, and above all it has bearing on the meta-issue of which issues are significant. For the two sides will, or ought to, recognize quite different questions as the central unsolved problems in the theory of modality.

For those on the realist side, the old problem of the ultimate source of our knowledge of modality remains, even if it is granted that the proximate source lies in knowledge of linguistic conventions. For knowledge of linguistic conventions constitutes knowledge of a reality independent of us only insofar as our linguistic conventions reflect, at least to some degree, such an ultimate reality. So for the realist the problem remains of explaining how such degree of correspondence as there is between distinctions in language and distinctions in the world comes about. If the distinction in the world is something primary and independent, and not a mere projection of the distinction in language, then how the distinction in language comes to be even imperfectly aligned with the distinction in the world remains to be explained. For it cannot be said that we have faculties responsive to modal facts independent of us – not in any sense of ‘responsive’ implying that if the facts had been different, then our language would have been different, since modal facts couldn’t have been different. What then is the explanation? This is the problem of the epistemology of modality as it confronts the realist, and addressing it is or ought to be at the top of the realist agenda.

As for the pragmatist side, a chief argument of thinkers from Kant to Ayer and Strawson and beyond for their anti-realist stance has been precisely that if the distinction we perceive in reality is taken to be merely a projection of a distinction created by ourselves, then the epistemological problem dissolves. That seems more like a reason for hoping the Kantian or Ayerite or Strawsonian view is the right one, than for believing that it is; but in any case, even supposing the pragmatist view is the right one, and the problems of the epistemology of modality are dissolved, still the pragmatist side has an important unanswered question of its own to address. The pragmatist account, begins by saying that we have certain reasons, connected with our various purposes in life, to use certain words, including ‘would’ and ‘might’, in certain ways, and thereby to make certain distinctions. What the pragmatist owes us is an account of what these purposes are, and how the rules of our language help us to achieve them. Addressing that issue is or ought to be at the top of the pragmatists’ to-do list.

While the positivist Ayer dismisses all metaphysics, the ordinary-language philosopher Strawson distinguishes good metaphysics, which he calls ‘descriptive’, from bad metaphysics, which he calls ‘revisionary’, but which rather be called ‘transcendental’ (without intending any specifically Kantian connotations). Descriptive metaphysics aims to provide an explicit account of our ‘conceptual scheme’, of the most general categories of commonsense thought, as embodied in ordinary language. Transcendental metaphysics aims to get beyond or behind all merely human conceptual schemes and representations to ultimate reality as it is in itself, an aim that Ayer and Strawson agree is infeasible and probably unintelligible. The descriptive/transcendental divide in metaphysics is a paradigmatically ‘metaphilosophical’ issue, one about what philosophy is about. Realists about modality are paradigmatic transcendental metaphysicians. Pragmatists must in the first instance be descriptive metaphysicians, since we must to begin with understand much better than we currently do how our modal distinctions work and what work they do for us, before proposing any revisions or reforms. And so the difference between realists and pragmatists goes beyond the question of what issue should come first on the philosopher’s agenda, being as it is an issue about what philosophical agendas are about.

The Mystery of Modality. Thought of the Day 78.0

The ‘metaphysical’ notion of what would have been no matter what (the necessary) was conflated with the epistemological notion of what independently of sense-experience can be known to be (the a priori), which in turn was identified with the semantical notion of what is true by virtue of meaning (the analytic), which in turn was reduced to a mere product of human convention. And what motivated these reductions?

The mystery of modality, for early modern philosophers, was how we can have any knowledge of it. Here is how the question arises. We think that when things are some way, in some cases they could have been otherwise, and in other cases they couldn’t. That is the modal distinction between the contingent and the necessary.

How do we know that the examples are examples of that of which they are supposed to be examples? And why should this question be considered a difficult problem, a kind of mystery? Well, that is because, on the one hand, when we ask about most other items of purported knowledge how it is we can know them, sense-experience seems to be the source, or anyhow the chief source of our knowledge, but, on the other hand, sense-experience seems able only to provide knowledge about what is or isn’t, not what could have been or couldn’t have been. How do we bridge the gap between ‘is’ and ‘could’? The classic statement of the problem was given by Immanuel Kant, in the introduction to the second or B edition of his first critique, The Critique of Pure Reason: ‘Experience teaches us that a thing is so, but not that it cannot be otherwise.’

Note that this formulation allows that experience can teach us that a necessary truth is true; what it is not supposed to be able to teach is that it is necessary. The problem becomes more vivid if one adopts the language that was once used by Leibniz, and much later re-popularized by Saul Kripke in his famous work on model theory for formal modal systems, the usage according to which the necessary is that which is ‘true in all possible worlds’. In these terms the problem is that the senses only show us this world, the world we live in, the actual world as it is called, whereas when we claim to know about what could or couldn’t have been, we are claiming knowledge of what is going on in some or all other worlds. For that kind of knowledge, it seems, we would need a kind of sixth sense, or extrasensory perception, or nonperceptual mode of apprehension, to see beyond the world in which we live to these various other worlds.

Kant concludes, that our knowledge of necessity must be what he calls a priori knowledge or knowledge that is ‘prior to’ or before or independent of experience, rather than what he calls a posteriori knowledge or knowledge that is ‘posterior to’ or after or dependant on experience. And so the problem of the origin of our knowledge of necessity becomes for Kant the problem of the origin of our a priori knowledge.

Well, that is not quite the right way to describe Kant’s position, since there is one special class of cases where Kant thinks it isn’t really so hard to understand how we can have a priori knowledge. He doesn’t think all of our a priori knowledge is mysterious, but only most of it. He distinguishes what he calls analytic from what he calls synthetic judgments, and holds that a priori knowledge of the former is unproblematic, since it is not really knowledge of external objects, but only knowledge of the content of our own concepts, a form of self-knowledge.

We can generate any number of examples of analytic truths by the following three-step process. First, take a simple logical truth of the form ‘Anything that is both an A and a B is a B’, for instance, ‘Anyone who is both a man and unmarried is unmarried’. Second, find a synonym C for the phrase ‘thing that is both an A and a B’, for instance, ‘bachelor’ for ‘one who is both a man and unmarried’. Third, substitute the shorter synonym for the longer phrase in the original logical truth to get the truth ‘Any C is a B’, or in our example, the truth ‘Any bachelor is unmarried’. Our knowledge of such a truth seems unproblematic because it seems to reduce to our knowledge of the meanings of our own words.

So the problem for Kant is not exactly how knowledge a priori is possible, but more precisely how synthetic knowledge a priori is possible. Kant thought we do have examples of such knowledge. Arithmetic, according to Kant, was supposed to be synthetic a priori, and geometry, too – all of pure mathematics. In his Prolegomena to Any Future Metaphysics, Kant listed ‘How is pure mathematics possible?’ as the first question for metaphysics, for the branch of philosophy concerned with space, time, substance, cause, and other grand general concepts – including modality.

Kant offered an elaborate explanation of how synthetic a priori knowledge is supposed to be possible, an explanation reducing it to a form of self-knowledge, but later philosophers questioned whether there really were any examples of the synthetic a priori. Geometry, so far as it is about the physical space in which we live and move – and that was the original conception, and the one still prevailing in Kant’s day – came to be seen as, not synthetic a priori, but rather a posteriori. The mathematician Carl Friedrich Gauß had already come to suspect that geometry is a posteriori, like the rest of physics. Since the time of Einstein in the early twentieth century the a posteriori character of physical geometry has been the received view (whence the need for border-crossing from mathematics into physics if one is to pursue the original aim of geometry).

As for arithmetic, the logician Gottlob Frege in the late nineteenth century claimed that it was not synthetic a priori, but analytic – of the same status as ‘Any bachelor is unmarried’, except that to obtain something like ‘29 is a prime number’ one needs to substitute synonyms in a logical truth of a form much more complicated than ‘Anything that is both an A and a B is a B’. This view was subsequently adopted by many philosophers in the analytic tradition of which Frege was a forerunner, whether or not they immersed themselves in the details of Frege’s program for the reduction of arithmetic to logic.

Once Kant’s synthetic a priori has been rejected, the question of how we have knowledge of necessity reduces to the question of how we have knowledge of analyticity, which in turn resolves into a pair of questions: On the one hand, how do we have knowledge of synonymy, which is to say, how do we have knowledge of meaning? On the other hand how do we have knowledge of logical truths? As to the first question, presumably we acquire knowledge, explicit or implicit, conscious or unconscious, of meaning as we learn to speak, by the time we are able to ask the question whether this is a synonym of that, we have the answer. But what about knowledge of logic? That question didn’t loom large in Kant’s day, when only a very rudimentary logic existed, but after Frege vastly expanded the realm of logic – only by doing so could he find any prospect of reducing arithmetic to logic – the question loomed larger.

Many philosophers, however, convinced themselves that knowledge of logic also reduces to knowledge of meaning, namely, of the meanings of logical particles, words like ‘not’ and ‘and’ and ‘or’ and ‘all’ and ‘some’. To be sure, there are infinitely many logical truths, in Frege’s expanded logic. But they all follow from or are generated by a finite list of logical rules, and philosophers were tempted to identify knowledge of the meanings of logical particles with knowledge of rules for using them: Knowing the meaning of ‘or’, for instance, would be knowing that ‘A or B’ follows from A and follows from B, and that anything that follows both from A and from B follows from ‘A or B’. So in the end, knowledge of necessity reduces to conscious or unconscious knowledge of explicit or implicit semantical rules or linguistics conventions or whatever.

Such is the sort of picture that had become the received wisdom in philosophy departments in the English speaking world by the middle decades of the last century. For instance, A. J. Ayer, the notorious logical positivist, and P. F. Strawson, the notorious ordinary-language philosopher, disagreed with each other across a whole range of issues, and for many mid-century analytic philosophers such disagreements were considered the main issues in philosophy (though some observers would speak of the ‘narcissism of small differences’ here). And people like Ayer and Strawson in the 1920s through 1960s would sometimes go on to speak as if linguistic convention were the source not only of our knowledge of modality, but of modality itself, and go on further to speak of the source of language lying in ourselves. Individually, as children growing up in a linguistic community, or foreigners seeking to enter one, we must consciously or unconsciously learn the explicit or implicit rules of the communal language as something with a source outside us to which we must conform. But by contrast, collectively, as a speech community, we do not so much learn as create the language with its rules. And so if the origin of modality, of necessity and its distinction from contingency, lies in language, it therefore lies in a creation of ours, and so in us. ‘We, the makers and users of language’ are the ground and source and origin of necessity. Well, this is not a literal quotation from any one philosophical writer of the last century, but a pastiche of paraphrases of several.