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Concept

Kripke model

A set of worlds with arrows between them, together with an assignment saying which statements hold at each world. It gives the modal operators their meaning: necessary at a world means true at every world that world points to.

Named by 9 essays across one field — each of them below, with the objects they name alongside it.

A set, its negation, and its double negation. Four bars on one number line showing an open set, its negation, their union, and the double negation.

The middle that is not excluded

Either it is raining or it is not. Drop that as an axiom and what is left is still a logic — one with models made of open sets and of stages of knowledge, in which a set and its negation between them miss the boundary.

logic · Non classical logic Which axioms hold on which frames. A table of frames against modal axioms, each cell decided by checking the axiom under every valuation.

The axiom is the shape of the graph

Add one operator meaning necessarily and the choice of which axioms to accept stops being a matter of taste. Each candidate axiom is true of exactly those worlds-and-arrows diagrams whose arrows have a stated property, and a logic is a class of graphs.

logic · Modal logic Two models the modal language cannot separate, and two it can. Four Kripke models in two pairs: the upper pair joined by a bisimulation and agreeing on every formula, the lower pair separated by a formula found by search.

Two diagrams the language cannot tell apart

A modal formula sees a diagram of worlds and arrows through a very narrow window. Exactly how narrow is settled by a game: where one player can answer every move, no formula whatever separates the two starting worlds, however different the diagrams look.

logic · Modal logic A model whose worlds are sets of sentences. Worlds labelled by which of a fixed finite set of formulas they accept, with an arrow wherever every boxed formula accepted by one has its inside accepted by the other.

Worlds built out of sentences

A Kripke model needs worlds, and nothing so far has said where worlds come from. They can be made of the syntax: a world is a set of formulas it commits to, one world sees another when the boxed commitments line up, and in the model that results every formula is true exactly where it was assumed.

logic · Modal logic Axioms, the conditions on the arrows they answer to, and the one that answers to none. A table of modal axioms with the property of the accessibility relation each corresponds to, every row decided by sweeping all relations on up to four worlds.

The axiom with no property of the arrows

Each axiom of modal logic can be matched by hand to a condition on the arrows between worlds. There is a recipe that does it for a whole class of axioms, and there is an axiom the recipe cannot reach — not because nobody has looked, but because no condition on the arrows defines it at all.

logic · Modal logic The frames on which provability makes sense, and the two that are refused. Six small frames marked by whether Löb's axiom is valid on them: transitive frames with no cycles accept it, and any frame in which a world can reach itself does not.

Necessity that means provable

Read the box as "the theory proves" and one modal logic stops being a proposal about what necessity might mean. It becomes a complete description of what a formal system can prove about its own proofs — and its frames run forward, compose, and stop.

logic · Modal logic A chain of 18 worlds, and the 3 the formulas can tell apart. A row of 18 circles for the worlds of the model, shaded by which of the 3 classes each falls into, above the quotient model's 3 worlds with the arrows the collapse gives them.

How many worlds a formula can need

A modal formula can be true in a model with infinitely many worlds. It can also be true in a small one — and the small one is built from the large one by throwing away every distinction the formula was never able to make.

logic · Modal logic 4 axioms against 4 finite algebras. A table of candidate axioms against finite Heyting algebras built from small orders, marking which algebras validate which axiom at every valuation.

Not one step but a continuum

Classical logic is the constructive system plus one axiom, which makes it sound as though there are two logics and one gap. There are uncountably many logics in that gap, each one a class of algebras, and the smallest separations between them fit in five elements.

logic · Non classical logic Gluing two countermodels: p ∨ ¬p. Two small Kripke models, each refuting one half of a disjunction, and the model made by placing both above a new first stage, at which neither half is forced.

A proof that says which half

Classical logic proves p ∨ ¬p without any idea which half is true. The constructive system never does that: whenever it proves a disjunction, it proves one of the two halves. The reason is a picture — two countermodels placed side by side above a new first stage that forces neither — and the logics between the two lose the property exactly when their pictures are not allowed to be glued.

logic · Non classical logic

Named alongside it

The objects these essays reach for when they reach for this one.

Modal logicExhaustive searchAccessibilityDecision procedureExcluded middleFrameHeyting algebraIntuitionistic logicAxiomBisimulationCompletenessConstructive proof

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