A logical argument can be examined in two ways: soundness and validity. You may have heard these two words used interchangeably, but for someone who may find interest in logic and philosophy, it is important to know the difference between the two terms. Let’s look at how we can determine what a valid argument is and then see how the soundness of an argument will determine how debatable the argument will be.
The validity of a deductive argument is simple: if the premises are true, then the conclusion must be true. Notice that this is a specific definition concerning the relationship between the premises and the conclusion. This rule doesn’t consider whether or not the premises, or the conclusion for that matter, are really true in real life. The validity of an argument isn’t concerned about whether the premises reflect reality- it is only concerned with what would be true if those premises reflected what was real. For example, we could come up with ridiculous premises and still yield a valid argument:
- If I flew to school on Monday, then you own 10 unicorns.
- I did fly to school on Monday.
- Therefore, you own 10 unicorns.
As ludicrous of an argument as this is, it is a valid argument (because it follows the Modus Ponens structure). If the argument is valid, then what is so silly about it? Well, we all know that unicorns don’t exist in the real world. We also know that since I can’t fly in real life, that I didn’t fly to school on Monday. So, we can reasonably say that the conditional premise 1) in and of itself isn’t really true and premise 2) also doesn’t match reality either. This means that the argument is not a sound argument, even though it is logically valid.
In contrast to the validity of an argument, the soundness of an argument considers whether the premises really reflect the way things are in our own world. We can see by inspection that usually the soundness of a premise is the main focus of debate in any given argument. It is usually easier to determine whether an argument is invalid than it is to determine whether an argument is sound. For example, we can say something like this below:
- If objective moral goodness exists, then God exists.
- Objective moral goodness does exist.
- Therefore, God does exist.
The argument above is simple and valid, however, both premise 1) and premise 2) can and have been debated. Since the argument is valid, the the conclusion is fixed and must be true if 1) and 2) are true. Therefore, 1) and 2) must be proven unsound to attack the argument. For example, someone who doesn’t believe this argument is sound may attack premise 1) by saying that just because objective moral goodness exists, doesn’t mean that it must come from God. Further, one may also object to premise 2) by disagreeing that objective moral goodness really exists.
In closing, it is crucial to remember the distinction between a sound argument and a valid argument. Just because an argument is valid doesn’t mean that it is necessarily sound. When valid arguments are challenged, they are typically attacked by arguing against the soundness of their premises. Therefore, when constructing a valid argument, it is critical to anticipate challenges to the soundness of one’s premises.