Quick Overview: Validity in Logic

In everyday speech to say something, especially an argument, is “valid,” means that it is reasonable and has a sound basis in logic and/or fact. However, in logic something being valid means that the argument’s conclusion must follow (is entailed) by the argument’s premises. Note, that validity (more fully called deductive validity) does not affect the truth of an argument, that is, an argument can be invalid but true, or valid but untrue.

A valid argument would be:

All dogs are animals

Purple is a dog

Therefore, Purple is an animal.

This argument is valid because the conclusion (“Purple is an animal“) must logically follow from the premises.

Another example of a valid argument would be:

All planets are flat,

Earth is a planet,

Therefore, Earth is flat.

Despite being utterly false, this argument is because the conclusion is entailed by the premise.

Remember, validity does not equate to truth, merely that a conclusion is necessitated by the premises.