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Logic
Logic (from the Ancient Greek: λογική, translit. logikḗ), is the systematic study of the form of valid inference, and the most general laws of truth. A valid inference is one where there is a specific relation of logical support between the assumptions of the inference and its conclusion. In ordinary discourse, inferences may be signified by words such as therefore, hence, ergo, and so on.
There is no universal agreement as to the exact scope and subject matter of logic (see § Rival conceptions, below), but it has traditionally included the classification of arguments, the systematic exposition of the 'logical form' common to all valid arguments, the study of proof and inference, including paradoxes and fallacies, and the study of syntax and semantics. Historically, logic has been studied in philosophy (since ancient times) and mathematics (since the mid19th century), and recently logic has been studied in computer science, linguistics, psychology, and other fields.
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A prolific writer, he was also a populariser of philosophy and a commentator on a large variety of topics, ranging from very serious issues to those much less so. Continuing a family tradition in political affairs, he was a prominent antiwar activist, championing free trade between nations and antiimperialism.
Did you know?
 ...that the Law of noncontradiction is the same as the Law of contradiction?
 ...that NAND alone or NOR alone make a functionally complete set of logical operators?
 ...that according to the continuum hypothesis, there is no cardinal number in between aleph null and the cardinality of the continuum?
 ...that all 24 valid syllogistic forms have names like 'Camestros' and 'Felapton'?
 ...that if a statement P implies another statement Q, and a third statement R also implies Q, and either P or R is true; then Q has to be true?
 ...that the dunce cap was named after a logician?
 ...that the collective noun for a group of logicians is a "sequitur of logicians"
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 ^ Godel, Escher, Bach: An Eternal Golden Braid, Douglas Hofstadter
 ^ Because nonwellformed formulas are rarely considered, some authors ignore them altogether. For these authors, "formula" and "wellformed formula" are synonyms. Other authors use the term "formula" for any string of symbols in the language; certain of these strings are then singled out as the wellformed formulas.