See Part 1 here. (I advise you to read part 1 first.)
One reason that not all good arguments are logically sound is because good arguments used in science are inductive, and inductive arguments are not meant to be logically sound. However, not all good deductive arguments are logically sound either. There is a sense that deductive arguments would ideally be logically sound, but some deductive arguments have sufficiently justified premises, even if those premises aren’t known to be true for certain. A good deductive argument must be logically valid, and it must have sufficiently justified premises. Even so, not all good deductive arguments are logically sound.
A good way to know if good deductive arguments can fail to be logically sound is to consider a good deductive argument that can fail to be logically sound. An uncontroversial example of a good argument is the following:
- All men are mortal.
- Socrates is a man.
- Therefore, Socrates is mortal.
Is this argument logically sound? Probably, but maybe not. The premises are sufficiently justified, but that doesn’t mean we know they are true for certain. If the premises are true, then it’s sound. If the premises are false, then it is not sound. We need to know why we would believe the conclusion is true. The reason that we should believe the conclusion is true is because the premises are well-justified, not because we know the argument is sound.
Consider the first premise in particular. Perhaps there is an immortal man who has kept his immortality a secret. If we found out a man is immortal, then the argument will no longer be a good argument. However, it would not be reasonable to require all good deductive arguments to have premises we know are true for absolute certain. Right now the premise is sufficiently justified and will remain so until we have significant counter-evidence against it. Imagine that all good deductive arguments had to use premises that we know are true for certain. In that case scientific conclusions could never be used for the premises of deductive arguments. Our best science concludes that all men are mortal, but sometimes scientific conclusions are discovered to be wrong. It would be absurd to say that no good deductive argument could use scientific conclusions as premises. Our best scientific conclusions are sufficiently justified and uncontroversial good deductive arguments can use our best scientific conclusions as premises.
Examples of scientific conclusions are “if the laws of nature will still exist tomorrow, then the law of gravity will still help us make predictions tomorrow” and “the laws of nature will still exist tomorrow.” A perfectly good deductive argument that uses these conclusions as premises is the following:
- The laws of nature will still exist tomorrow.
- If the laws of nature will still exist tomorrow, then the law of gravity will still help us make predictions tomorrow.
- Therefore, the law of gravity will help us make predictions tomorrow.
We don’t know that these premises are true for absolutely certain. Sometimes the conclusions of science are proven wrong at some later point. The laws of nature might not exist tomorrow. Even so, it is perfectly reasonable to assume they will. This is a good argument, but it might not be logically sound.
We can summarize my argument that not all deductive arguments are logically sound as the following:
- We know that the above deductive arguments are good arguments, but we don’t know for certain that they are logically sound. They are good arguments whether or not they are logically sound.
- If they are good arguments whether or not they are logically sound, then not all good deductive arguments are logically sound.
- Therefore, not all good deductive arguments are logically sound.
In conclusion, not all good deductive arguments are logically sound. We hope our good deductive arguments are logically sound, but sometimes they aren’t. Instead, I suggest that we define good deductive arguments as those that are logically valid with sufficiently justified premises.
Update (8/23/2012): I added the following clarification concerning why the first argument is good and why it doesn’t need to be logically sound — “We need to know why we would believe the conclusion is true. The reason that we should believe the conclusion is true is because the premises are well-justified, not because we know the argument is sound.” Additionally, I added a summary of my argument. Finally, someone replied with a lengthy email who seems to misunderstand this essay because he/she didn’t see part 1. If you strongly disagree or feel confused, make sure to read part 1.
- Good Arguments (I disagree with the author that good arguments must have true premises, and that good deductive arguments must be logically sound.)
- Not All Good Arguments Are Logically Sound (Part 1)