B If the truth of a proposition can be established in more than one way, the corresponding connective has multiple introduction rules.   E B New logics are usually formalised in a general type theoretic setting, known as a logical framework. In logic and proof theory, natural deduction is a kind of proof calculus in which logical reasoning is expressed by inference rules closely related to the "natural" way of reasoning. Labels also allow the naming of worlds in Kripke semantics; Simpson (1993) presents an influential technique for converting frame conditions of modal logics in Kripke semantics into inference rules in a natural deduction formalisation of hybrid logic. Following the standard approach, proofs are specified with their own formation rules for the judgment "π proof". ∧ These logical frameworks are themselves always specified as natural deduction systems, which is a testament to the versatility of the natural deduction approach.  true ∧ ⊃ B Thus, a natural deduction proof does not have a purely bottom-up or top-down reading, making it unsuitable for automation in proof search. C Dual to introduction rules are elimination rules to describe how to deconstruct information about a compound proposition into information about its constituents. The simplest possible proof is the use of a labelled hypothesis; in this case the evidence is the label itself. I ∧ This is read as: if falsehood is true, then any proposition C is true. A A {\displaystyle {\frac {A{\hbox{ true}}\qquad B{\hbox{ true}}}{(A\wedge B){\hbox{ true}}}}\ \wedge _{I}}.  true Stouppa (2004) surveys the application of many proof theories, such as Avron and Pottinger's hypersequents and Belnap's display logic to such modal logics as S5 and B. 3) Look for the premise which includes the letters that make up the conclusion. To introduce conjunctions, i.e., to conclude "A and B true" for propositions A and B, one requires evidence for "A true" and "B true". Dually, local completeness says that the elimination rules are strong enough to decompose a connective into the forms suitable for its introduction rule.  true C ⋮ ∧  The term natural deduction (or rather, its German equivalent natürliches Schließen) was coined in that paper: Ich wollte nun zunächst einmal einen Formalismus aufstellen, der dem wirklichen Schließen möglichst nahe kommt.  true I Like logic, type theory has many extensions and variants, including first-order and higher-order versions. In the sequent calculus all inference rules have a purely bottom-up reading. Normalisability is a rare feature of most non-trivial type theories, which is a big departure from the logical world. We write "Ω;Γ ⊢ A true" where Γ contains the true hypotheses as before, and Ω contains valid hypotheses. This is a demo of a proof checker for Fitch-style natural deduction systems found in many popular introductory logic textbooks. Propositions in the logical interpretation are now viewed as types, and proofs as programs in the lambda calculus.  Thus "it is raining" is a judgment, which is evident for the one who knows that it is actually raining; in this case one may readily find evidence for the judgment by looking outside the window or stepping out of the house. To give an example, consider disjunction; the right rules are familiar: Recall the ∨E rule of natural deduction in localised form: The proposition A ∨ B, which is the succedent of a premise in ∨E, turns into a hypothesis of the conclusion in the left rule ∨L. ⋮  It is much easier to show this indirectly by means of a cut-free sequent calculus presentation. Gentzen's original treatment of excluded middle prescribed one of the following three (equivalent) formulations, which were already present in analogous forms in the systems of Hilbert and Heyting: (XM3 is merely XM2 expressed in terms of E.) This treatment of excluded middle, in addition to being objectionable from a purist's standpoint, introduces additional complications in the definition of normal forms. As an example of the use of inference rules, consider commutativity of conjunction. The letter A stands for any expression representing a proposition; the truth judgments thus require a more primitive judgment: "A is a proposition". The interpretation is: "B true is derivable from A ∧ (B ∧ C) true". E B 2 ¬ The following diagram summarises the change. Dependent type theory in full generality is very powerful: it is able to express almost any conceivable property of programs directly in the types of the program. ( A Go back to problems you've already done and do them again. II", "On the meanings of the logical constants and the justifications of the logical laws", "A judgmental reconstruction of modal logic". The left rule, however, performs some additional substitutions that are not performed in the corresponding elimination rules.  true Just keep plugging away.  true The logical connectives are also given a different reading: conjunction is viewed as product (×), implication as the function arrow (→), etc.