philosophy journal

philosophy journal

For this journal assignment, determine if each of following arguments validity (valid or invalid) and soundness (sound or unsound). In addition, explain WHY each argument is valid or invalid, sound or unsound. Here is an example:

1. No mammals are birds. Some penguins are mammals. Therefore, some penguins are not birds.

This argument is valid, because the form, if all premises are true, guarantees that the conclusion is also true. However, this argument is unsound, because one of the premises (some penguins are mammals) is not true.

Each question below will be worth two points each; one point if you correctly identify the validity/soundness, and one point if you correctly explain why. Partial credit will be given only one of the two criteria is correctly identified.

QUESTIONS

1. Some twins are sisters. All twins are siblings. Therefore, some siblings are sisters.
2. All students are dormitory residents. No dormitory residents are birds. Therefore, no birds are students.
3. All millionaires are rich people. Some Australians are not rich people. Therefore, some Australians are not millionaires.
4. No spider monkeys are elephants. All elephants are animals. Therefore, no spider monkeys are animals.
5. Some tacos have cheese. Some tacos have lettuce. Therefore, some tacos are burritos.

Solution Preview

1. Some twins are sisters. All twins are siblings. Therefore, some siblings are sisters.

This argument is valid since it fulfills the form whereby if all premises are true the conclusion is also true. All the premises are true thereby making the argument valid. Moreover, the argument is sound as all premises are not only true but also due to the fact that the use of the word ‘Some” (Some twins are sisters, some siblings are sisters) acknowledges that while some twins may be sisters some may be male and thus brothers therefore not all siblings are sisters.

On the other hand, if the premise (all twins are siblings) means that all twins in the world are siblings that premise is wrong making the argument invalid. However, since the conclusion is true and rational the argument would still remain sound.

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