# Validity Validity All UH students are communists All

• Slides: 21

Validity

Validity All UH students are communists. All communists like broccoli. All UH students like broccoli.

Validity All UH students are communists. All communists like broccoli. GOOD All UH students like broccoli.

Validity All UH students are communists. All communists like broccoli. GOOD All UH students like broccoli. Logic does not try to decide which statements are true. The issue for logic is: Is the REASONING good?

Validity All UH students are communists. All communists like broccoli. All UH students like broccoli. Premises Conclusion Validity Defined: 1. Assuming the premises are (all) T, the conclusion has to be T. 2. It is impossible for the premises to be T, and the conclusion to be F.

Validity and Truth All UH students are communists. False All communists like broccoli. False All UH students like broccoli. False The statements in the argument are FALSE but… The argument is VALID. Valid Defined: 1. Assuming the premises are (all) T, the conclusion has to be T. 2. It is impossible for the premises to be T, and the conclusion to be F.

Validity and Truth All canaries have feathers. True Some pets are canaries. True

Validity and Truth All canaries have feathers. True Some pets are canaries. True The statements in the argument are TRUE but… The argument is INVALID.

Validity and Truth All canaries have feathers. True Some pets are canaries. True The statements in the argument are TRUE but… The argument is INVALID. Validity Defined: 1. Assuming the premises are (all) T, the conclusion has to be T. 2. It is impossible for the premises to be T, and the conclusion to be F.

Validity and Truth IMAGINE this: Canaries just never ended up being pets; but parrots and mynas are still pets. So it is possible for the premises to be T, and the conclusion F. All canaries have feathers. True Some pets have feathers. True Some canaries are pets. False

Validity and Truth All UH students are communists. F All canaries have feathers. T All communists like broccoli. F Some pets have feathers. T All UH students like broccoli. F Some canaries are pets. Do Not Ask: Are the statements T? Ask Instead: Assuming the premises are T, must the conclusion be T? Validity Defined: 1. Assuming the premises are (all) T, the conclusion has to be T. 2. It is impossible for the premises to be T, and the conclusion to be F. T

Validity and Argument Form Logic is the study of Argument Form not Truth. All UH students are Communists. F All Us are Cs. All Communists like Broccoli. F All Cs are Bs. All UH students like Broccoli. F All Us are Bs. Any argument with this form will be valid, regardless of how false (or silly) its premises may be.

Validity and Argument Form Logic is the study of Argument Form not truth. All Canaries have Feathers. T All Cs are Fs. Some Pets have Feathers. T Some Ps are Fs. Some Canaries are Pets. T Some Cs are Ps. Any argument with this form will be invalid, regardless of how true its premises may be. Just because all Cs are Fs and some Ps are Fs, It does not follow that some Cs are Ps.

Validity and Argument Form Logic is the study of Argument Form. All Cats have Feet. T All Cs are Fs. Some Pigs have Feet. T Some Ps are Fs. Some Cats are Pigs. F Some Cs are Ps. Just because all Cs are Fs and some Ps are Fs, It does not follow that some Cs are Ps.

Validity is Different From Truth It is crucial that you distinguish validity from truth. All canaries have feathers. T All Cs are Fs. Some pets have feathers. T Some Ps are Fs. Some canaries are pets. T Some Cs are Ps.

Validity is Different From Truth It is crucial that you distinguish validity from truth. All canaries have feathers. T All Cs are Fs. Some pets have feathers. T Some Ps are Fs. Some canaries are pets. T Some Cs are Ps. INVALID Invalid arguments may have true conclusions.

Validity is Different From Truth It is crucial that you distinguish validity from truth. All UH students are communists. F All Us are Ws. All communists like broccoli. F All Ws are B. All UH students like broccoli. F All Us are B.

Validity is Different From Truth It is crucial that you distinguish validity from truth. All UH students are communists. F All Us are Ws. All communists like broccoli. F All Ws are B. All UH students like broccoli. F All Us are B. VALID Valid arguments may have false conclusions.

Validity is Different From Truth It is crucial that you distinguish validity from truth. The truth values of the statements in an argument do not allow you to predict whether it is valid. With one exception … If the premises are T and the conclusion is F, Then the argument has to be invalid. Valid Defined: 1. Assuming the premises are (all) T, the conclusion has to be T. 2. It is impossible for the premises to be T, and the conclusion to be F.

A Common Mistake It is crucial that you distinguish validity from truth. When I ask for the definition of validity, students say: The premises are T and the conclusion is T. Valid Defined: 1. Assuming the premises are (all) T, the conclusion has to be T. 2. It is impossible for the premises to be T, and the conclusion to be F.

A Common Mistake It is crucial that you distinguish validity from truth. When I ask for the definition of validity, students say: The premises are T and the conclusion is T. WRONG! If the premises are T then the conclusion must be T. RIGHT! For more click here