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Critical Thinking: An Introduction to the Basic Skills, 4th ed., by William Hughes and Jonathan Lavery (Broadview Press, 2004)


In a deductive argument, the principle that if all the premises are true, the conclusion must also be true.

See also:


From the Latin, "strong, potent"

Examples and Observations:

  • "A formally valid argument that has true premises is said to be a sound argument. In debate or discussion, therefore, an argument may be attacked in two ways: by attempting to show that one of its premises is false or by attempting to show that it is invalid. On the other hand, if one concedes the truth of the premises of a formally valid argument, one must also concede the truth of the conclusion--or be guilty of irrationality."
    (Martin P. Golding, Legal Reasoning. Broadview Press, 2001)

  • "I once heard former RIBA President Jack Pringle defend flat roofs with the following syllogism: We all like Edwardian terraces. Edwardian terraces use curtain walls to hide their sloping roofs and pretend they're flat. Ergo: we must all like flat roofs.

    "Except that we don't, and they still leak."
    (Jonathan Morrison, "My Top Five Architectural Pet Hates." The Guardian, Nov. 1, 2007)

  • "There are a great many valid argument forms, but we shall consider only four basic ones. They are basic in the sense that they occur in everyday use, and that all other valid argument forms can be derived from these four forms:

    1. Affirming the Antecedent
      If p then q
      Therefore, q

    2. Denying the Consequent
      If p then q
      Therefore, not-p

    3. Chain Argument
      If p then q
      If q then r
      Therefore, if p then r

    4. Disjunctive Syllogism
      Either p or q
      Therefore, q
    Whenever we find an argument whose form is identical to one of these valid argument forms, we know that it must be a valid argument."
    (William Hughes and Jonathan Lavery, Critical Thinking: An Introduction to the Basic Skills. Broadview Press, 2004)
Pronunciation: vah-LI-di-tee
Also Known As: formal validity, valid argument
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