History-based models

We start by taking any non-empty set of atomic and unanalysed events Σ. An infinite ‘history’ is just an unending sequence of events:

Definition 7.2.2 (Infinite History): H is an infinite history over Σ iffdf H PΣω.

We use the following notation: WherekPω,HkHæ pk 1q. Unsurprisingly, we call

such finite sequences ‘finite histories’. Definition 7.2.3 (Finite History):

A finite history of length kis a sequence xσ1. . . σky P Σk. The set of all finite histories

is denoted Σ. We write the concatenation of two finite histories h xσ1. . . σmy and

h1_xσ1₁. . . σ_n1_y as h"h1, _xsigma1. . . σmσ11. . . σ1ny. We abbreviate xey to e. The empty

history is denoted 1.

Given such a set of events Σ, we also specify a protocol _„ Σ, which can be thought of as thelegal sequences of events. For example, a game of chess might be analysed by saying that there is a set of events ΣC, which correspond to the moves permitted: for

example, moving a pawn two steps forward; but certain sequences of events (e.g. two white pieces moving in succession) might be disallowed by the rules, and so would not, in this analysis, be part of the protocol. It is of course a metaphysical question whether we might insist that a protocol applies outside the realm of games, in the real world: Definition 7.2.4 (Binary Protocol):

Pis abinary protocol over Σ iffdf P„Σω.

An idea behind the history-based structures of [24] is that for each agent some events are ‘visible’ and others ‘invisible’. So they define what we will call ‘infinite history systems’ as follows:

S xP,tEauaPn, Vyis an n-agent history system over Σ iffdf Pis a binary protocol over

Σ and_@a_Pn, E_a„Σ.

Here eachEa represents the events of which agentacan be, and always is, aware of the

occurrence. EachE_a then induces a local view function between finite histories; we first define inductively the projection function of an agent on the basis ofEa. The projection

of hfor ais just the events ofh thatacan see, in the order in which they occur inh: λap1q 1

λ_aph"e_q λ_aph_q" "

e if e_PEa

1 otherwise

[24] calls the range of a’s projection function a’s ‘local histories’. There is a sense in which a’s local history ath (i.e. λaphq) represents the way in which agentasees h: She

has seen those events of which she can be aware, and is entirely ignorant of those of which she cannot be aware. We say ‘entirely ignorant’ because it is not the case thata can be correctly characterised by saying that she thinks that some events have not taken place. Rather, those events are, as it were, beyond her ken: She is unable to entertain the possibility of them having occurred. There is a sense in which they are not part of her environment. This should become clearer when we definea’s local view function on the basis of the projection function. That will be a function between finite histories allowed by the protocol and sets of such finite histories. It will be useful to have some additional notation: We define the relation Ã_„2Σ

2Σω : Definition 7.2.6 (Finite Prefix):

For a finite history h and a (finite or infinite) historyH, we say that h is a finite prefix of H, writtenhÃH, iff_{df D}H1 :Hh"H1.

Then the function F inP re: 2ΣYΣω

Ñ 2Σ_{, giving the set of finite prefixes of a set of} histories:

Definition 7.2.7 (Finite Prefixes): F inP repYiHiq Yith|DH:h"H Hiu

Now we can define the local view function induced by a projection function: faphq: F inP repPq Ñ 2F inP repPq

h _{ÞÑ t}h1_|λ_aph_q λ_aph1_qu

That is, the points (histories) which agent adoes not rule out are precisely those that appear the same to her as the actual history. Thus the agent can never rule out the actual history, which will always appear to her as it appears to her. This is what we meant by saying that the agent is entirely ignorant of those events of which she is not

aware, and it entails that such events do not lead her to false beliefs simply because she is not aware of their occurrence.

Definition 7.2.8 (n-agent History Structure):

S xP,tfauaPny is ann-agent history structure over Σ iffdf Pis a binary protocol over

Σ_@a_Pn, f_a:F inP re_pP_{q Ñ}2F inP repPq_.

Clearly an n-agent history system defines uniquely an n-agent history structure. Fur- thermore, given an n-agent history structure defined by some n-agent history system, thatn-agent history system is uniquely determined. Notice that if we set things up dif- ferently, allowing ourselves to define directly theλa function (as is done in [23]), rather

than via theE_i functions, then we would not be able to retrieve that λ_i function if we were given only the induced local view function. All that is important from the point of view ofa, or at least from the point of view of her epistemic state, is that she has some way of distinguishing between histories, even if she ‘sees’ them as different from the way they are. All that she needs in order to be omniscient, then, is forλi to be bijective. We

will later allow thefafunctions to be defined independently, without reference to anyEi

function or toλi’s; before that, we look at the way these structures are used as the mod-

els for a formal language. In order to make the structures models for a language whose propositional variables are Φ, they must be supplemented with a valuation function: Definition 7.2.9 (n-agent History Model):

xP,tfauaPn, Vyis ann-agent history model over Σ iffdf xP,tfauaPnyis ann-agent history

structure over Σ andV :F inP repPq ÑΦ.

[24] introduce two temporal modal operators into their languageLT: A unary operator

O, meaning ‘at the next instant’, and a binary operatorU, whereφU ψis to be interpreted asφ holds untilψ is true. This necessitates giving the semantics of that language with respect not to finite histories but to infinite histories indexed by an ordinal giving the position in that history, i.e. tuplesxH, ky. This is because for example the finite historyh does not contain enough information to express the intended meaning ofOpbecause its truth depends on what the next state is, which is not determined byh. This expressive power is however never used in [24], so after stating the semantics provided there, we will follow up and make formal a remark made by the authors to the effect that one can evaluate most such formulae with respect to finite histories rather than infinite histories supplemented by ordinals. Again, we suppress the subscript M in xH, ky |ùM φ. The

first four clauses in the following truth-condition definition are essentially that same as those given in 2.3.1.

1. _xH, k_{y |ù}p_ôp_PV_pH_kq 2. _xH, k_{y |ù} φ_{ô x}H, k_{y *}φ

4. xH, ky |ùBaφô @xH1, k1y:H_k11 PfapHkq,xH1, k1y |ùφ

5. xH, ky |ùOφô xH, k 1y |ùφ

6. xH, ky |ùφU ψ ô Dm¥k@l¥kpl mñ pxH, ly |ùφ^ xH, my |ùψqq