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Ideas of M Fitting/R Mendelsohn, by Text

[American, fl. 1998, Two logicians working in New York.]

1998 First-Order Modal Logic
Pref p.-1 'Predicate abstraction' abstracts predicates from formulae, giving scope for constants and functions
     Full Idea: 'Predicate abstraction' is a key idea. It is a syntactic mechanism for abstracting a predicate from a formula, providing a scoping mechanism for constants and function symbols similar to that provided for variables by quantifiers.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], Pref)
1.10 p.25 F: will sometime, P: was sometime, G: will always, H: was always
     Full Idea: We introduce four future and past tense operators: FP: it will sometime be the case that P. PP: it was sometime the case that P. GP: it will always be the case that P. HP: it has always been the case that P. (P itself is untensed).
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.10)
     A reaction: Temporal logic begins with A.N. Prior, and starts with □ as 'always', and ◊ as 'sometimes', but then adds these past and future divisions. Two different logics emerge, taking □ and ◊ as either past or as future.
1.11 p.28 In epistemic logic knowers are logically omniscient, so they know that they know
     Full Idea: In epistemic logic the knower is treated as logically omniscient. This is puzzling because one then cannot know something and yet fail to know that one knows it (the Principle of Positive Introspection).
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.11)
     A reaction: This is nowadays known as the K-K Problem - to know, must you know that you know. Broadly, we find that externalists say you don't need to know that you know (so animals know things), but internalists say you do need to know that you know.
1.11 p.28 Read epistemic box as 'a knows/believes P' and diamond as 'for all a knows/believes, P'
     Full Idea: In epistemic logic we read Υ as 'KaP: a knows that P', and ◊ as 'PaP: it is possible, for all a knows, that P' (a is an individual). For belief we read them as 'BaP: a believes that P' and 'CaP: compatible with everything a believes that P'.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.11)
     A reaction: [scripted capitals and subscripts are involved] Hintikka 1962 is the source of this. Fitting and Mendelsohn prefer □ to read 'a is entitled to know P', rather than 'a knows that P'.
1.12.2 Ex p.34 □P → P is not valid in D (Deontic Logic), since an obligatory action may be not performed
     Full Idea: System D is usually thought of as Deontic Logic, concerning obligations and permissions. □P → P is not valid in D, since just because an action is obligatory, it does not follow that it is performed.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.12.2 Ex)
1.2 p.5 Modality affects content, because P→◊P is valid, but ◊P→P isn't
     Full Idea: P→◊P is usually considered to be valid, but its converse, ◊P→P is not, so (by Frege's own criterion) P and possibly-P differ in conceptual content, and there is no reason why logic should not be widened to accommodate this.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.2)
     A reaction: Frege had denied that modality affected the content of a proposition (1879:p.4). The observation here is the foundation for the need for a modal logic.
1.3 p.5 Modal logic adds □ (necessarily) and ◊ (possibly) to classical logic
     Full Idea: For modal logic we add to the syntax of classical logic two new unary operators □ (necessarily) and ◊ (possibly).
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.3)
1.5 p.9 We let 'R' be the accessibility relation: xRy is read 'y is accessible from x'
     Full Idea: We let 'R' be the accessibility relation: xRy is read 'y is accessible from x'.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.5)
1.5 p.9 Modern modal logic introduces 'accessibility', saying xRy means 'y is accessible from x'
     Full Idea: Modern modal logic takes into consideration the way the modal relates the possible worlds, called the 'accessibility' relation. .. We let R be the accessibility relation, and xRy reads as 'y is accessible from x.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.5)
     A reaction: There are various types of accessibility, and these define the various modal logics.
1.6 p.12 A 'model' is a frame plus specification of propositions true at worlds, written < G,R,||- >
     Full Idea: A 'model' is a frame plus a specification of which propositional letters are true at which worlds. It is written as , where ||- is a relation between possible worlds and propositional letters. So Γ ||- P means P is true at world Γ.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.6)
1.6 p.12 A 'frame' is a set G of possible worlds, with an accessibility relation R, written < G,R >
     Full Idea: A 'frame' consists of a non-empty set G, whose members are generally called possible worlds, and a binary relation R, on G, generally called the accessibility relation. We say the frame is the pair so that a single object can be talked about.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.6)
1.6 p.12 The symbol ||- is the 'forcing' relation; 'Γ ||- P' means that P is true in world Γ
     Full Idea: The symbol ||- is used for the 'forcing' relation, as in 'Γ ||- P', which means that P is true in world Γ.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.6)
1.6 p.12 Each line of a truth table is a model
     Full Idea: Each line of a truth table is, in effect, a model.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.6)
     A reaction: I find this comment illuminating. It is being connected with the more complex models of modal logic. Each line of a truth table is a picture of how the world might be.
1.6 p.13 If a proposition is possibly true in a world, it is true in some world accessible from that world
     Full Idea: If a proposition is possibly true in a world, then it is also true in some world which is accessible from that world. That is: Γ ||- ◊X ↔ for some Δ ∈ G, ΓRΔ then Δ ||- X.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.6)
1.6 p.13 If a proposition is necessarily true in a world, it is true in all worlds accessible from that world
     Full Idea: If a proposition is necessarily true in a world, then it is also true in all worlds which are accessible from that world. That is: Γ ||- □X ↔ for every Δ ∈ G, if ΓRΔ then Δ ||- X.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.6)
1.7 p.17 Accessibility relations can be 'reflexive' (self-referring), 'transitive' (carries over), or 'symmetric' (mutual)
     Full Idea: A relation R is 'reflexive' if every world is accessible from itself; 'transitive' if the first world is related to the third world (ΓRΔ and ΔRΩ → ΓRΩ); and 'symmetric' if the accessibility relation is mutual.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.7)
     A reaction: The different systems of modal logic largely depend on how these accessibility relations are specified. There is also the 'serial' relation, which just says that any world has another world accessible to it.
1.8 p.19 The system D has the 'serial' conditon imposed on its accessibility relation
     Full Idea: The system D has the 'serial' condition imposed on its accessibility relation - that is, every world must have some world which is accessible to it.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
1.8 p.19 The system T has the 'reflexive' conditon imposed on its accessibility relation
     Full Idea: The system T has the 'reflexive' condition imposed on its accessibility relation - that is, every world must be accessible to itself.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
1.8 p.19 The system K4 has the 'transitive' condition on its accessibility relation
     Full Idea: The system K4 has the 'transitive' condition imposed on its accessibility relation - that is, if a relation holds between worlds 1 and 2 and worlds 2 and 3, it must hold between worlds 1 and 3. The relation carries over.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
1.8 p.19 The system S4 has the 'reflexive' and 'transitive' conditions on its accessibility relation
     Full Idea: The system S4 has the 'reflexive' and 'transitive' conditions imposed on its accessibility relation - that is, every world is accessible to itself, and accessibility carries over a series of worlds.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
1.8 p.19 System S5 has the 'reflexive', 'symmetric' and 'transitive' conditions on its accessibility relation
     Full Idea: The system S5 has the 'reflexive', 'symmetric' and 'transitive' conditions imposed on its accessibility relation - that is, every world is self-accessible, and accessibility is mutual, and it carries over a series of worlds.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
     A reaction: S5 has total accessibility, and hence is the most powerful system (though it might be too powerful).
1.8 p.19 The system K has no accessibility conditions
     Full Idea: The system K has no frame conditions imposed on its accessibility relation.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
     A reaction: The system is named K in honour of Saul Kripke.
1.8 p.19 The system B has the 'reflexive' and 'symmetric' conditions on its accessibility relation
     Full Idea: The system B has the 'reflexive' and 'symmetric' conditions imposed on its accessibility relation - that is, every world must be accessible to itself, and any relation between worlds must be mutual.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
2.2 p.48 The prefix σ names a possible world, and σ.n names a world accessible from that one
     Full Idea: A 'prefix' is a finite sequence of positive integers. A 'prefixed formula' is an expression of the form σ X, where σ is a prefix and X is a formula. A prefix names a possible world, and σ.n names a world accessible from that one.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
2.2 p.48 Conj: a) if σ X∧Y then σ X and σ Y b) if σ (X∧Y) then σ X or σ Y
     Full Idea: General tableau rules for conjunctions: a) if σ X ∧ Y then σ X and σ Y b) if σ (X ∧ Y) then σ X or σ Y
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
2.2 p.48 Bicon: a)if σ(X↔Y) then σ(X→Y) and σ(Y→X) b) [not biconditional, one or other fails]
     Full Idea: General tableau rules for biconditionals: a) if σ (X ↔ Y) then σ (X → Y) and σ (Y → X) b) if σ (X ↔ Y) then σ (X → Y) or σ (Y → X)
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
2.2 p.49 Universal: a) if σ ◊X then σ.m X b) if σ □X then σ.m X [m exists]
     Full Idea: General tableau rules for universal modality: a) if σ ◊ X then σ.m X b) if σ □ X then σ.m X , where m refers to a world that can be seen (rather than introducing a new world).
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
     A reaction: Note that the universal rule of □, usually read as 'necessary', only refers to worlds which can already be seen, whereas possibility (◊) asserts some thing about a new as yet unseen world.
2.2 p.49 Existential: a) if σ ◊X then σ.n X b) if σ □X then σ.n X [n is new]
     Full Idea: General tableau rules for existential modality: a) if σ ◊ X then σ.n X b) if σ □ X then σ.n X , where n introduces some new world (rather than referring to a world that can be seen).
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
     A reaction: Note that the existential rule of ◊, usually read as 'possibly', asserts something about a new as yet unseen world, whereas □ only refers to worlds which can already be seen,
2.2 p.49 Implic: a) if σ (X→Y) then σ X and σ Y b) if σ X→Y then σ X or σ Y
     Full Idea: General tableau rules for implications: a) if σ (X → Y) then σ X and σ Y b) if σ X → Y then σ X or σ Y
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
2.2 p.49 Negation: if σ X then σ X
     Full Idea: General tableau rule for negation: if σ X then σ X
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
2.2 p.49 Disj: a) if σ (X∨Y) then σ X and σ Y b) if σ X∨Y then σ X or σ Y
     Full Idea: General tableau rules for disjunctions: a) if σ (X ∨ Y) then σ X and σ Y b) if σ X ∨ Y then σ X or σ Y
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.2)
2.3 p.52 D serial: a) if σ □X then σ ◊X b) if σ ◊X then σ □X
     Full Idea: System D serial rules (also for T, B, S4, S5): a) if σ □X then σ ◊X b) if σ ◊X then σ □X
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.3)
2.3 p.52 B symmetric: a) if σ.n □X then σ X b) if σ.n ◊X then σ X [n occurs]
     Full Idea: System B symmetric rules (also for S5): a) if σ.n □X then σ X b) if σ.n ◊X then σ X [where n is a world which already occurs]
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.3)
2.3 p.52 T reflexive: a) if σ □X then σ X b) if σ ◊X then σ X
     Full Idea: System T reflexive rules (also for B, S4, S5): a) if σ □X then σ X b) if σ ◊X then σ X
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.3)
2.3 p.52 4 transitive: a) if σ □X then σ.n □X b) if σ ◊X then σ.n ◊X [n occurs]
     Full Idea: System 4 transitive rules (also for K4, S4, S5): a) if σ □X then σ.n □X b) if σ ◊X then σ.n ◊X [where n is a world which already occurs]
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.3)
2.3 p.52 4r rev-trans: a) if σ.n □X then σ □X b) if σ.n ◊X then σ ◊X [n occurs]
     Full Idea: System 4r reversed-transitive rules (also for S5): a) if σ.n □X then σ □X b) if σ.n ◊X then σ ◊X [where n is a world which already occurs]
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.3)
2.3 p.54 S5: a) if n ◊X then kX b) if n □X then k X c) if n □X then k X d) if n ◊X then k X
     Full Idea: Simplified S5 rules: a) if n ◊X then kX b) if n □X then k X c) if n □X then k X d) if n ◊X then k X. 'n' picks any world; in a) and b) 'k' asserts a new world; in c) and d) 'k' refers to a known world
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 2.3)
4.3 p.87 □ must be sensitive as to whether it picks out an object by essential or by contingent properties
     Full Idea: If □ is to be sensitive to the quality of the truth of a proposition in its scope, then it must be sensitive as to whether an object is picked out by an essential property or by a contingent one.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 4.3)
     A reaction: This incredibly simple idea strikes me as being powerful and important. ...However, creating illustrative examples leaves me in a state of confusion. You try it. They cite '9' and 'number of planets'. But is it just nominal essence? '9' must be 9.
4.5 p.93 Counterpart relations are neither symmetric nor transitive, so there is no logic of equality for them
     Full Idea: The main technical problem with counterpart theory is that the being-a-counterpart relation is, in general, neither symmetric nor transitive, so no natural logic of equality is forthcoming.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 4.5)
     A reaction: That is, nothing is equal to a counterpart, either directly or indirectly.
4.5 p.93 A 'constant' domain is the same for all worlds; 'varying' domains can be entirely separate
     Full Idea: In 'constant domain' semantics, the domain of each possible world is the same as every other; in 'varying domain' semantics, the domains need not coincide, or even overlap.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 4.5)
4.9 p.113 The Barcan says nothing comes into existence; the Converse says nothing ceases; the pair imply stability
     Full Idea: The Converse Barcan says nothing passes out of existence in alternative situations. The Barcan says that nothing comes into existence. The two together say the same things exist no matter what the situation.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 4.9)
     A reaction: I take the big problem to be that these reflect what it is you want to say, and that does not keep stable across a conversation, so ordinary rational discussion sometimes asserts these formulas, and 30 seconds later denies them.
6.3 p.136 The Barcan corresponds to anti-monotonicity, and the Converse to monotonicity
     Full Idea: The Barcan formula corresponds to anti-monotonicity, and the Converse Barcan formula corresponds to monotonicity.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 6.3)
7.1 p.141 The Indiscernibility of Identicals has been a big problem for modal logic
     Full Idea: Equality has caused much grief for modal logic. Many of the problems, which have struck at the heart of the coherence of modal logic, stem from the apparent violations of the Indiscernibility of Identicals.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 7.1)
     A reaction: Thus when I say 'I might have been three inches taller', presumably I am referring to someone who is 'identical' to me, but who lacks one of my properties. A simple solution is to say that the person is 'essentially' identical.
7.3 p.148 Objects retain their possible properties across worlds, so a bundle theory of them seems best
     Full Idea: The property of 'possibly being a Republican' is as much a property of Bill Clinton as is 'being a democrat'. So we don't peel off his properties from world to world. Hence the bundle theory fits our treatment of objects better than bare particulars.
     From: M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 7.3)
     A reaction: This bundle theory is better described in recent parlance as the 'modal profile'. I am reluctant to talk of a modal truth about something as one of its 'properties'. An objects, then, is a bundle of truths?