Assuming that we have established that we have an actual argument, the next step is to 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.
If a deductive argument is valid, then that means that the reasoning process behind the inferences is correct and that no fallacies have been committed. 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, then that means that not only are all the inferences true, but the premises are also true. Hence, the conclusion is necessarily true. Here are two examples to 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 this demonstrates 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. As explained above, because its form is valid and its premises are true, we are guaranteed that the conclusion is also true.
Evaluating Inductive Arguments
With inductive arguments, on the other hand, they are considered strong if the conclusion follows probably 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 that we 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 didnt care about the factors mentioned (perhaps you suffer from allergies and dont 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 that there have 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. It must be remembered, 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 that has been demonstrated is that the argument itself cannot be used to establish the truth of the conclusion.
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, this does not mean that they are. If you think that they are (or may be) false, you can challenge them and ask for support. This would require the other person to create a new argument in which the old premises become the conclusions.
If the inferences and reasoning process in an argument are false, that will usually be because some fallacy has been committed. A fallacy is an error in the reasoning process whereby the connection between the premises and the conclusion is not what has been claimed. There are numerous types of fallacies which are described and defined in this FAQ.