FALSE PREMISE.

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Presentation transcript:

FALSE PREMISE

A statement is any unambiguous declarative sentence about a fact (or non-fact) about the world. It says that something is (or isn’t) the case. An argument is a series of statements meant to establish a claim. A claim or conclusion is the statement whose truth an argument is meant to establish.

A statement’s truth value is either true or false. All statements have a truth value. A statement is false when what it says about the world is not actually the case. A statement is true when what it says about the world is actually the case. A premise is a statement that is used in an argument to establish a conclusion.

What we can say about an argument: An argument is valid if its premises necessarily lead to its conclusion. That is, if you accept that the premises are all true, you must accept that the conclusion is true. An argument is sound if it is valid and you accept that all its premises are true. A good, convincing argument is a sound argument. That is, since you accept all the premises are true, you must accept the conclusion is true (because the argument is valid). A bad argument is any other kind of argument.

Examples If Barack Obama is President of the USA, then Michelle Obama is First Lady. Barack is President of the USA. Therefore, Michelle is First Lady. This argument is valid. Material Implication: The truth of (A) is sufficient by itself for the truth of the consequent (B).

Invalid (false premises, true conclusion) We can IMAGINE that the premises are true and the conclusion is false even if they are actually just the opposite. ALL invalid arguments are UNSOUND.

Invalid (false premises, true conclusion) Consider: The president of the United States must be 100 years of age or older. George W. Bush must be 100 years of age or older. So, George W. Bush is president of the United States.

Invalid (false premises, true conclusion) If George W. Bush is president of the United States, then George W. Bush must be 100 years of age or older. George W. Bush must be 100 years of age or older. So, George W. Bush is president of the United States.

Invalid (false premises, true conclusion) For either example, the logic is invalid and the premises are false. For the premises to be true, all of them need to be true. But, for the premises to be false, only one need be false. So, an argument with a mixture of true and false premises is still considered to be an argument with false premises--it is false that all of the premises are true. Nevertheless, in these examples, the conclusion is true.

FALSE PREMISE OR NOT? “Every animal needs to breathe in order to live. Fish are animals. Fish cannot breathe in the air. Therefore, fish cannot live in the air.” Here, the claim is that “fish cannot live in the air.” The premises are “Every animal needs to breathe in order to live,” “Fish are animals,” and “Fish cannot breathe in the air.” The argument is valid – the premises necessarily lead to the conclusion. The argument is also sound – the premises are true. It is a good argument.

FALSE PREMISE OR NOT? Oranges are green. All green things make me sick. Therefore, oranges make me sick.” The claim is “oranges make me sick.” The premises are “Oranges are green,” and “All green things make me sick.” The argument is valid – if we accept the premises, we are forced to accept the conclusion. However, the argument is not sound – oranges are not, in fact, green, so one of the premises is false. This is a bad argument.

FALSE PREMISE OR NOT? “Broccoli is green. Some green things make me sick. Therefore, broccoli makes me sick.” The claim is “broccoli makes me sick.” The premises are “Broccoli is green,” and “Some green things make me sick.” Here, all the premises are true. However, the argument is not valid – even if we accept the premises, we are not forced to accept the conclusion. Just because some green things are sickening does not mean that broccoli is. This is a bad or unsound argument.

FALSE PREMISE OR NOT? “Whales know how to play hockey. Therefore, Canadians like winter.” The claim is “Canadians like winter.” The premise is “Whales know how to play hockey.” The argument is neither valid nor sound. It’s a bad argument. (Again, it doesn’t make any difference whether the conclusion is true.)