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Proof theory for modal logic
Proof theory for modal logic

... applicability does not depend on any global condition that would involve a survey of the whole derivation; They are simply rules with a particular condition on the context of their premisses, in this case that the context be empty. Sequent calculi for the modal logics K, T, S4, and S5 are presented ...
CPSC 2105 Lecture 6 - Edward Bosworth, Ph.D.
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A Paedagogic Example of Cut-Elimination
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... in proving that F is valid and found a formal proof in LI′ (which can be routinely transformed to a proof in LI). Otherwise, no reduction branch of P is a proof; this is the case iff in every reduction branch there is at least one topmost formula of the form x ≤ y where x and y are distinct variable ...
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... Let us look at the examples considered earlier. In the first one there are two variables and the minterms occurring in the expression correspond to 0, 2 and 3 and this is written as 0, 2, 3. In the second example the minterms correspond to 7, 6, 3, 1 respectively and the expression is written as 1 ...
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A Propositional Modal Logic for the Liar Paradox Martin Dowd
A Propositional Modal Logic for the Liar Paradox Martin Dowd

Chapter 2.6 Notes: Prove Statements about Segments and Angles
Chapter 2.6 Notes: Prove Statements about Segments and Angles

... • A proof is a logical argument that shows a satement is true. Two-Column Proofs: • A two-column proof has numbered statements and corresponding reasons that show an argument in a logical order. • In a two-column proof, each statement in the lefthand column is either given information or the result ...
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Law of thought

The laws of thought are fundamental axiomatic rules upon which rational discourse itself is often considered to be based. The formulation and clarification of such rules have a long tradition in the history of philosophy and logic. Generally they are taken as laws that guide and underlie everyone's thinking, thoughts, expressions, discussions, etc. However such classical ideas are often questioned or rejected in more recent developments, such as Intuitionistic logic and Fuzzy Logic.According to the 1999 Cambridge Dictionary of Philosophy, laws of thought are laws by which or in accordance with which valid thought proceeds, or that justify valid inference, or to which all valid deduction is reducible. Laws of thought are rules that apply without exception to any subject matter of thought, etc.; sometimes they are said to be the object of logic. The term, rarely used in exactly the same sense by different authors, has long been associated with three equally ambiguous expressions: the law of identity (ID), the law of contradiction (or non-contradiction; NC), and the law of excluded middle (EM).Sometimes, these three expressions are taken as propositions of formal ontology having the widest possible subject matter, propositions that apply to entities per se: (ID), everything is (i.e., is identical to) itself; (NC) no thing having a given quality also has the negative of that quality (e.g., no even number is non-even); (EM) every thing either has a given quality or has the negative of that quality (e.g., every number is either even or non-even). Equally common in older works is use of these expressions for principles of metalogic about propositions: (ID) every proposition implies itself; (NC) no proposition is both true and false; (EM) every proposition is either true or false.Beginning in the middle to late 1800s, these expressions have been used to denote propositions of Boolean Algebra about classes: (ID) every class includes itself; (NC) every class is such that its intersection (""product"") with its own complement is the null class; (EM) every class is such that its union (""sum"") with its own complement is the universal class. More recently, the last two of the three expressions have been used in connection with the classical propositional logic and with the so-called protothetic or quantified propositional logic; in both cases the law of non-contradiction involves the negation of the conjunction (""and"") of something with its own negation and the law of excluded middle involves the disjunction (""or"") of something with its own negation. In the case of propositional logic the ""something"" is a schematic letter serving as a place-holder, whereas in the case of protothetic logic the ""something"" is a genuine variable. The expressions ""law of non-contradiction"" and ""law of excluded middle"" are also used for semantic principles of model theory concerning sentences and interpretations: (NC) under no interpretation is a given sentence both true and false, (EM) under any interpretation, a given sentence is either true or false.The expressions mentioned above all have been used in many other ways. Many other propositions have also been mentioned as laws of thought, including the dictum de omni et nullo attributed to Aristotle, the substitutivity of identicals (or equals) attributed to Euclid, the so-called identity of indiscernibles attributed to Gottfried Wilhelm Leibniz, and other ""logical truths"".The expression ""laws of thought"" gained added prominence through its use by Boole (1815–64) to denote theorems of his ""algebra of logic""; in fact, he named his second logic book An Investigation of the Laws of Thought on Which are Founded the Mathematical Theories of Logic and Probabilities (1854). Modern logicians, in almost unanimous disagreement with Boole, take this expression to be a misnomer; none of the above propositions classed under ""laws of thought"" are explicitly about thought per se, a mental phenomenon studied by psychology, nor do they involve explicit reference to a thinker or knower as would be the case in pragmatics or in epistemology. The distinction between psychology (as a study of mental phenomena) and logic (as a study of valid inference) is widely accepted.
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