Critiquing Arguments

How To Tell When Arguments Are Valid or Sound

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Once you have established that you have an actual argument, you should examine it for validity. There are two points on which an argument might fail: its premises or its inferences. Because of this, it is necessary to distinguish between valid arguments and sound arguments.

Valid vs. Sound Arguments

If a deductive argument is valid, that means the reasoning process behind the inferences is correct and there are no fallacies. If the premises of such an argument are true, then it is impossible for the conclusion not to be true. Conversely, if an argument is invalid, then the reasoning process behind the inferences is not correct.

If a deductive argument is sound, that means that not only are all the inferences true, but the premises are also true. Hence, the conclusion is necessarily true. Two examples illustrate the differences between a valid and a sound argument.

  1. All birds are mammals. (premise)
  2. A platypus is a bird. (premise)
  3. Therefore, the platypus is a mammal. (conclusion)

This is a valid deductive argument, even though the premises are both false. But because those premises are not true, the argument is not sound. It is interesting to note that the conclusion is true, which shows that an argument with false premises can nevertheless produce a true conclusion.

  1. All trees are plants. (premise)
  2. The redwood is a tree. (premise)
  3. Therefore, the redwood is a plant. (conclusion)

This is a valid deductive argument because its form is correct. It is also a sound argument because the premises are true. Because its form is valid and its premises are true, the conclusion is guaranteed to be true.

Evaluating Inductive Arguments

Inductive arguments, on the other hand, are considered strong if the conclusion probably follows from the premises and weak if it follows only improbably from the premises, despite what is claimed about it. If the inductive argument is not only strong but also has all true premises, then it is called cogent. Weak inductive arguments are always uncogent. Here is an example:

Strolling through the woods is usually fun. The sun is out, the temperature is cool, there is no rain in the forecast, the flowers are in bloom, and the birds are singing. Therefore, it should be fun to take a walk through the woods now.

Assuming you care about those premises, then the argument is strong. Assuming that the premises are all true, then this is also a cogent argument. If we didn’t care about the factors mentioned (perhaps you suffer from allergies and don’t like it when the flowers are in bloom), it would be a weak argument. If any of the premises turned out to be false (for example, if it is actually raining), then the argument would be uncogent. If additional premises turned up, like there have been reports of a bear in the area, then that would also make the argument uncogent.

To critique an argument and show that it is invalid or possibly unsound or uncogent, it is necessary to attack either the premises or the inferences. Remember, however, that even if it can be demonstrated that both the premises and the intermediate inferences are incorrect, that does not mean that the final conclusion is also false. All you have demonstrated is that the argument itself cannot be used to establish the truth of the conclusion.

Premises Are Assumed True

In an argument, the premises offered are assumed to be true, and no effort is made to support them. But, just because they are assumed to be true, does not mean that they are. If you think they are (or may be) false, you can challenge them and ask for support. The other person would need to create a new argument in which the old premises become the conclusions.

If the inferences and reasoning process in an argument is false, that's usually because of some fallacy. A fallacy is an error in the reasoning process whereby the connection between the premises and the conclusion is not what has been claimed.