Contradictory Premises in an Argument

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Contradictory premises involve an argument (generally considered a logical fallacy) that draws a conclusion from inconsistent or incompatible premises.

Essentially, a proposition is contradictory when it asserts and denies the same thing.

Examples and Observations:

  • "'Here’s an example of Contradictory Premises: If God can do anything, can He make a stone so heavy that He won’t be able to lift it?'

    "'Of course,' she replied promptly.

    "'But if He can do anything, He can lift the stone,' I pointed out.

    "'Yeah,' she said thoughtfully. 'Well, then I guess He can’t make the stone.'

    "'But He can do anything,' I reminded her.

    "She scratched her pretty, empty head. 'I’m all confused,' she admitted.

    "'Of course you are. Because when the premises of an argument contradict each other, there can be no argument. If there is an irresistible force, there can be no immovable object. If there is an immovable object, there can be no irresistible force. Get it?'

    "'Tell me more of this keen stuff,' she said eagerly."
    (Max Shulman, The Many Loves of Dobie Gillis. Doubleday, 1951)
  • "It is . . . sometimes difficult to distinguish between real and apparent incompatible premises. For example, a father who is trying to convince his child that no one should be trusted is obviously making an exception of himself. If he really were making incompatible claims ('since you should trust no one, and you should trust me'), no rational conclusion could or should be drawn by the child. However, the incompatible premises are only apparent; the father has carelessly overstated the first premise. If he had said, 'Don't trust most people' or 'Trust very few people,' or 'Don't trust anyone except me,' he would have had no trouble avoiding the contradiction."
    (T. Edward Damer, Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments, 6th ed. Wadsworth, 2008)
  • "To say that lying is justified must, according to the rational principle enshrined in the categorical imperative, be to say that everyone is justified in lying. But the implication of this is that the distinction between lying and telling the truth is no longer valid. If lying is universalized (i.e., if 'everyone ought to lie' becomes a universal maxim of action), then the whole rationale for lying disappears because nobody will consider that any response might be truthful. Such a [maxim] is self-contradictory, since it negates the distinction between lying and truth-telling. Lying can exist only if we expect to hear the truth; if we expect to be told lies, the motive for lying disappears. To identify lying as ethical, then, is to be inconsistent. It is to try to sustain two contradictory premises ('everyone ought to lie' and 'everyone ought to tell the truth') and is therefore not rational."
    (Sally E. Talbot, Partial Reason: Critical and Constructive Transformations of Ethics and Epistemology. Greenwood, 2000)
  • Contradictory Premises in Mental Logic
    "Unlike the standard logic of textbooks, people draw no conclusions from contradictory premises--such premise sets cannot qualify as assumptions. No one ordinarily would assume a contradictory set of premises, but would see such as absurd."
    (David P. O'Brien, "Mental Logic and Irrationality: We Can Put a Man on the Moon, So Why Can't We Solve These Logical Reasoning Problems." Mental Logic, ed. by Martin D. S. Braine and David P. O'Brien. Lawrence Erlbaum, 1998)
  • "In standard logic an argument is valid as long as there is no assignment of truth values to its atomic propositions such that the premises taken conjunctively are true and the conclusion is false; thus any argument with contradictory premises is valid. In mental logic nothing could be inferred in such a situation except that some assumption is wrong, and the schemas are not applied to premises unless the premises are accepted."
    (David P. O'Brien, "Finding Logic in Human Reasoning Requires Looking in the Right Places." Perspectives on Thinking and Reasoning, ed. by Stephen E. Newstead and Jonathan St.B. T. Evans. Lawrence Erlbaum, 1995)

Also Known As: Incompatible Premises