# What one may come to know

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1 ANALYSIS 64. APRIL 004 What one may come to know Johan van Benthem. Verificationism and the Fitch paradox The general verification thesis says that what is true can be known or formally: f Æ Kf VT A surprising and much-discussed argument by Fitch trivializes this principle. It uses just a weak modal epistemic logic to show that VT collapses the notions of truth and knowledge, by taking the following clever substitution instance for f: P & ÿkp Æ K(P & ÿkp) Then we have the following chain of three conditionals: (a) K(P & ÿkp) Æ (KP & KÿKP) in the minimal modal logic for the knowledge operator K, (b) (KP & KÿKP) Æ (KP & ÿkp) in the modal logic T, and so (c) (KP & ÿkp) Æ^in the minimal modal logic for. Thus, a contradiction has been derived from the assumption P & ÿkp, and we have shown overall that P implies KP, making truth and knowledge equivalent. Proposed remedies for the paradox fall mainly into two kinds (cf. Brogaard and Salerno 00; Wansing 00). Some weaken the logic in the argument still further. This is like turning down the volume on your radio so as not to hear the bad news. You will not hear much good news either. Other remedies leave the logic untouched, but weaken the verification principle itself. This is like censoring the news: you hear things loud and clear, but they may not be so interesting. The proposal made below falls into the latter category, but using a different perspective from mere tinkering with proof rules or premises. We will emphasize positive reasons why VT can, and sometimes should, fail having to do with the ways in which we learn new information.. Knowable propositions and learning Fitch s substitution instance exemplifies a much older problem about knowledge called Moore s Paradox. It consists in the observation that the statement Analysis 64., April 004, pp Johan van Benthem

2 96 johan van benthem P, but I don t know it can be true, but cannot be known, as K(P & ÿkp) evidently implies a contradiction. Now, in an epistemic logic for a single agent, the possible knowledge of a proposition f requires that Kf be satisfiable at some world in some model, and hence in all alternatives to that world. This differs from ordinary epistemic satisfiability, which just demands truth of f at some world in some model. Tennant 00 argues persuasively for the following restriction on the intended applications of VT to propositions f: Kf is consistent CK In simple epistemic S5-models, this special requirement amounts to global satisfiability of f: i.e. its truth throughout at least one model. Like ordinary satisfiability, this notion is decidable for most modal logics, and hence constraints of knowability can be formulated at least in an effective manner. But there is a bit more to the situation! Our first observation is that CK only partially captures the intuition behind VT. 3. A dynamic shift: consistent update Consider any epistemic model (M, s), with a designated world s standing for the actual situation. What might be known in this setting seems restricted, intuitively, to what might be known correctly about that actual situation. We know already that it is one of the worlds in M. What we might learn is that this model can be shrunk still further, zooming in on the location of s. In this dynamic sense, the verification principle that every true statement may be known amounts to stating that What is true may come to be known VT* Clearly, VT* only holds for propositions f that satisfy CK. But it is more demanding. We need the truth of Kf not in just any model, but in some submodel of the current one. Fact. CK does not imply VT* for all propositions f. Proof. Let < >f be the existential dual of the operator K, standing for the epistemic (not the earlier modal!) notion of holding it possible that f. Now consider the statement f = ( P& <> ÿp) KÿP The better-known version of Moore s Paradox rather has the doxastic form P, but I don t believe it. But Moore 96 does discuss a knowledge version as well. Still, the computational complexity of global decidability may go up from that for standard S5, as we are now adding a so-called universal modality. Cf. Blackburn, de Rijke and Venema 00.

3 what one may come to know 97 This is knowable in the sense of CK, since K((P & < >ÿp) KÿP) is consistent: it holds in a model consisting of just one world with ÿp, where the agent knows that ÿp. But here is a two-world S5-model M where f holds in the actual world, even though there is no truthful announcement that would ever make us learn that f: the actual world P some other world ÿp In the actual world, (P & < >ÿp) KÿP holds, but it fails in the other world. Hence, K((P & < >ÿp) KÿP) fails in the actual world. The only truthful proper update of this model M would just retain its actual world. But in the resulting one-world epistemic model with the proposition letter P true, K((P & < >ÿp) KÿP) fails. QED. Thus, consistency of Kf need not be enough if we wish to learn that f here and now in any model where it holds, as expressed by VT*. Our first point, then, is that In a natural learning scenario, the Verification Thesis places stronger requirements on propositions than those stated so far in the literature. This observation suggests a closer look at the general dynamic viewpoint underpinning VT*. In a nutshell, what we know is the result of actions of learning. 4. Epistemic logic dynamified The simplest way of learning is by being told through a true new proposition which prunes the current epistemic model. In particular, a public announcement f! of assertion f does not just evaluate f truthconditionally in the current model (M, s). It rather changes that model, by eliminating all those worlds from it which fail to satisfy f: from (M, s) s to s f (M f, s) ÿf

4 98 johan van benthem This scenario works for simple questions and answers, but also for more intricate puzzles of knowledge and ignorance (van Benthem 003). 3 Thus, the semantic setting for basic forms of learning is a family of epistemic models standing for the relevant information states, related by a repertoire of actions of announcing propositions that increase information by moving from one model to another. Complete systems for this dynamified epistemics mix standard epistemic logics with dynamic logics of actions, with expressions describing what holds after an action was performed: [a]y y holds after every successful execution of action a The expression a may be a computer program or some physical action, or a speech act. In particular, in this way, one can express and investigate systematic statements about epistemic effects of successful communication: [f!]k j y after a true public announcement of f, j knows that y There are complete and decidable logical calculi for this richer language extending epistemic S5 with suitable laws for actions. These include axioms relating statements about the result of an action to those that were true before. 4 In particular, epistemic logics for communication emphasize the multi-agent character of speakers, hearers and audiences. Accordingly, the language can iterate knowledge assertions, as in K ÿk P knows that does not know that P There are also new notions for groups of agents G, such as common knowledge C G f: everyone knows that f, and they also know that the others know, and so on to any finite depth of iteration of mutual knowledge. This point about epistemic interaction will return below, as what seem like paradoxes for the case of lonesome knowers may look brighter in groups. 5. The dynamic logic of learning One currently open issue in dynamic epistemic logics concerns the generic effects of public announcement. At first, this seems simple. Here is a puta- 3 More sophisticated product update formats, beyond simple elimination, model complex forms of communication mixing public actions and information hiding. This more general format covers much of what happens in games and more realistic communicative settings. 4 As an illustration, one typical valid principle reduces knowledge after communication to relativized knowledge that must be true before it: [A!] K i f (A Æ K i (A Æ [A!] f)).

6 00 johan van benthem from CDD 3 CCD 3 DDD DCD 3 DDC* CDC 3 DCC CCC to DDD 3 CDD DDC* DCD 3 CDC 3 CCD DCC When no one knows his status, the bottom worlds disappear: DDD 3 The final update is to CDD DCD DDC* DDC* With k muddy children, k rounds of public ignorance assertions are needed to achieve common knowledge about who is dirty, while the announcement that the muddy children know their status achieves common knowledge of the whole situation. Thus, assertions of ignorance can drive a positive process of gathering information, and their ability to invalidate themselves may even be the crowning event. Indeed, the last announcement of ignorance for the muddy children led to their knowing the actual world. Logicians have adapted to this situation, and turned a problem into an object of study. What we see in puzzles like this is merely that communication is more interesting than what the putative principle LP suggests. For instance, we can investigate what special syntactic forms of assertion do become common knowledge upon announcement. 5 And this again suggests more general classifications. Statements of atomic facts may be called selffulfilling: once announced, their common knowledge results. Moore s 5 For instance, all universal modal formulae are self-fulfilling. These are the ones constructed using atoms and their negations, conjunction, disjunction, K i and C G. But there are other self-fulfilling types of statement as well, such as < >p. A complete syntactic characterization of the self-fulfilling patterns has been one of the open modeltheoretic problems of epistemic dynamic logic since the mid-990s.

7 what one may come to know 0 statement, on the other hand, is self-refuting: once announced, its negation always becomes common knowledge. But there are also wavering statements in between. Given the analogy between VT and LP, one might also develop an analogous enriched verification logic, distinguishing different roles for different types of statement. In the remainder of this paper, we develop this technical theme a bit further. What does knowability or learnability of propositions amount to in a dynamic epistemic setting? 6. Exploring learnable propositions As in the usual discussion of the Knower paradox, consider the case of a single agent. Suggestions for the case of more agents will follow later. Define a learnable proposition f as one whose truth can always become known by announcement of some suitable true formula A. That is, the following implication is valid for such formulae: f Æ\$A <A!>Kf Learnability Here the existential modality <A!>Kf, dual to the above [A!]Kf, says that a truthful announcement of A is possible in the current model (M, s), leading to knowledge of f after the corresponding update. Note also that the consequent \$A<A!>Kf is shorthand for an infinite disjunction over all formulae A of our language. Fact. Learnability is decidable in S5. Proof. It is well-known that all models of an S5-language for a finite set of proposition letters can be finitely enumerated, as the only options that matter to truth are which propositional valuations occur in the set of worlds. Thus, for each epistemic formula f, we can enumerate all relevant models M, s = f in a finite list. Now, for f to be learnable in the above sense, each of the models (M, s) in that list must have a submodel N (again in the list) containing s with f true in every world of that submodel. It is immediate that this can be checked effectively. QED. A stricter form of learnability would demand more uniformly that there be some finite set of announcements A one of which must lead to knowledge of f in any given model of f. This learnability by finite cases is equivalent to the above version, however, by the compactness theorem for S5 or more simply, by the above enumeration argument. A truly stronger uniform version of the requirement is the existence of one single assertion A such that the following formula is valid: f Æ<A!>Kf Uniform learnability Fact. Uniform learnability is stronger than learnability. Proof. Consider the following 4-world epistemic S5-model M:

8 0 johan van benthem p, q 3 ÿ p, ÿ q p, ÿ q 4 ÿ p, q Let f be an epistemic formula which is only true in the following minimal situations: (a) in the pictured 4-world model M: at the worlds, 3, and no others (b) in the two smaller models indicated by the ellipses: at both worlds. It is easy to write down such a formula explicitly. 6 According to the above description (a), (b), f is learnable: some update makes it known whenever it is true. But it is clear no single formula A does this job uniformly, since the selection of the submodel in M has to depend on which of the two f-worlds is our point of departure. QED. Still stronger is the case where announcing f itself produces its knowledge. This is the earlier learning situation of self-fulfilling assertions, restricted to the single-agent case: f Æ<f!>Kf Self-fulfilment Fact. Statements can be uniformly learnable without being self-fulfilling. Proof. Consider the following model M, in the same style as the preceding one: p, q 3 ÿ p, ÿ q p, ÿ q Let f hold only in (a) in model M: at world, (b) in M s oval two-world submodel: at both worlds. 6 One describes a model M globally by stating the possibility of each valuation in it, viewed as a conjunction of atoms and their negations, and then taking a universal modality over the disjunction of all these. Statements picking out specific worlds s in the model can then be conjoined with this. The total formula is a disjunction of a number of such local descriptions for various (M, s).

9 what one may come to know 03 Uniform learnability is satisfied. In every model (M, s) where this formula f holds, announcing the true atomic fact p makes f true throughout the resulting model. But self-fulfilment fails. Announcing the true statement f itself in the 3-world initial model (M, ) would leave just the -world submodel p, q. The reason is that, since we are not in the smaller submodel, the parts of the disjunction f referring to the latter will be false. But in this one-world submodel {}, f fails by its definition. QED. The upshot of this more detailed analysis is as follows. VP and LP are analogous to some extent, but the two putative learning principles do not coincide. On the way to this conclusion, we have seen something positive: the flexibility of the dynamic framework in phrasing different versions of learnability. This concludes our account of the single agent setting for epistemic update and learning. Our third main point, then, is that VT, VT* and LP point toward an interesting general logic of knowledge assertions, announcements and learning actions. Looking at some possible extensions adds still further detail to this perspective. 7. Refining the issues Our analysis has looked at the Verification Thesis and the Paradox of the Knower in terms of epistemic actions. This does not solve the original problem, but places it in a broader setting of interaction between many agents. In particular, the original Paradox of the Knower now becomes a special case in several senses. First, even with one agent, two different senses of learnability emerged: either by means of fixed assertions, or by context-dependent assertions. But also, the multi-agent setting suggests further refinements. With a single agent, the only candidate for the required knowledge level was Kf. But now one can require knowledge for other agents as well: some, or all. For example, Moore s Paradox disappears with some other agent learning that P, and does not know it, as the iterated epistemic formula K (P & ÿk P) may quite well become true. Also, with groups, we can strengthen the original knowledge condition of VT as follows: If f is true, then it is possible that f becomes common knowledge. Perhaps each member finds out part of the truth, and, by pooling this information, they arrive at C G f. For example, consider the following model M with actual world p, q:

10 04 johan van benthem p, q ÿ p, q ÿ p, ÿ q Announcing q will make know the Moore statement that p and does not know it. But this can never become common knowledge in the group {, }. What can become common knowledge, however, is p & q, when announces that q, and then says p. Many further distinctions can be made in interactive versions of knowability. There may be a price for this expressive power, however, in that the above results about learnability may become harder to formulate and prove for many agents. 7 Also, more delicate learning scenarios involving secrecy, hiding, and even misleading, occur in epistemic update logic, with different subgroups getting different information about the facts: cf. Baltag, Moss and Solecki 998. Finally, the dynamic component of the logic also adds a dimension. In a piece of recent jargon, the phrase knowable suffers from the common disease called \$ sickness. This means using an existentially quantified notion in a situation where more explicit information is available, whose logic could be brought out. Common symptoms are frequent uses of -ility s. Compare: provability versus a concrete proof, past tense as once upon a time versus some specific past episode, solvability versus producing an algorithm, winnability of a game versus a concrete winning strategy, etc. A full-fledged dynamic epistemic logic would cure the sickness in the particular case of knowability by making learning actions and their properties an explicit part of the logic of verificationism, however construed. 8 7 Technically, a multi-agent epistemic language with a common knowledge modality is not like plain S5. Simple enumeration arguments like that for decidability of singleagent learnability no longer hold, and the same is true for other model-theoretic techniques. In particular, we do not know precisely how our earlier results of 6 on learnability fare in this setting. 8 As a side benefit, our proposal also enriches dynamic epistemic logic. Our observations about single-agent S5 show that learnability assertions are definable there, and do not add anything new to the language. But now consider public announcements A! in a first-order language, where a formula A = A(x) restricts the full domain to the definable subdomain of objects satisfying A. Now, expressive power may increase. For example, take any strict linear order satisfying the first-order theory of (N, <) which extends beyond N by adding copies of the integers Z. Its only first-order definable subsets of objects are the finite and the co-finite ones. Now consider the firstorder learnability assertion

11 what one may come to know Conclusion We have shown that knowability of a proposition involves more than consistency of its being known, by placing the Paradox of the Knower in a dynamic setting where learning involves changing the current epistemic model. The Verification Thesis then turns out related to the Learning Principle for public announcement. Elaborating this analogy, we found different versions of knowability in an update setting, plus interesting extensions to multi-agent learning. This twist in perspective also reflects a change in mood. Much of the literature on the Fitch Paradox seems concerned with averting a disaster, and saving as large a chunk of verificationism as possible from the clutches of inconsistency. In our perspective, there is no saving VT but there is also no such gloom. For in losing a principle, we gain a general logical study of knowledge and learning actions, and their subtle properties. The failure of naive verificationism just highlights the intriguing ways in which human communication works. University of Amsterdam and Stanford University Plantage Muidergracht 4, NL-08 TV Amsterdam The Netherlands References Baltag, A., L. Moss and S. Solecki The logic of public announcements, common knowledge and private suspicions. Proceedings TARK 998, Los Altos: Morgan Kaufmann Publishers. Updated version, Department of Cognitive Science, Indiana University, Bloomington, and Department of Computing, Oxford University Benthem, J. van. 00. One is a lonely number, the logic of communicative update. Invited lecture, Colloquium Logicum, Muenster 00. Report , Institute for Logic, Language and Information, University of Amsterdam. Blackburn, P., M. de Rijke, and Y. Venema. 00. Modal Logic. Cambridge: Cambridge University Press. Brogaard, B. and J. Salerno. 00. Fitch s paradox of knowability. Stanford Electronic Encyclopedia of Philosophy at Moore, G. E. 96. The Commonplace Book London: Allen & Unwin. Tennant, N. 00. Victor vanquished. Analysis 6: Wansing, H. 00. Diamonds are a philosopher s best friends. The knowability paradox and modal epistemic relevance logic. Journal of Philosophical Logic 3: Some true announcement makes the following true: the current object n is the greatest point, while every object different from zero has a predecessor. This can only be true for those objects n which lie at some finite distance from the zero of the model. These form an initial copy of N, which is not definable in firstorder logic. Balder ten Cate has pointed out that one might also do this argument in a temporal language, closer to the epistemic modal original.

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