Evaluation of Arguments

Evaluation of Arguments

1. 1. Establish exactly what the argument entails (identify premisses and conclusion)

2. 2. Establish whether the propositions are true

3. Test the validity

4. Based on these 3 it can be classified as true/false, valid/invalid or reliable/unreliable

5. I. Some VALID arguments contain ONLY TRUE propositions - true premisses and a true conclusion

   • All mammals have lungs. All whales are mammals. Therefor all whales have lungs.

6. II. Some VALID arguments contain ONLY FALSE propositions

   • All four-legged creatures have wings. All spiders have four legs. Therefor all spiders have wings.

7. III. Some INVALID arguments contain only true propositions - all their premisses are true and their conclusions too

   • If I owned all the gold in Fort Knox, then I would be wealthy. I do not own all the gold in Fort Knox. Therefor I am wealthy.

8. IV. Some INVALID arguments contain ONLY TRUE PREMISSES and have FALSE CONCLUSIONS.

   • If Rockefeller owned all the gold in F.K., the Rockefeller would be wealthy. R'Feller does not own all the gold in f.k. Therefor R'feller is not wealthy.

9. V. Some VALID arguments have FALSE premisses and a TRUE conclusion

   • All fish are mammals. All whales are fish. Therefor all whales are mammals.

10. VI. Some INVALID arguments also have FALSE PREMISSES and a TRUE CONCL.

    • All mammals have wings. All whales have wings. Therefor all whales are mammals.

11. VII. Some INVALID arguments contain all false propositions - false premisses and a false conclusion.

    • All mammals have wings. All whales have wings. Therefor all mammals are whales.