Sound and unsound arguments

Once we know an argument is valid, more work needs to be done. This is because we cannot know the conclusion yet, since we have not established whether the reasons, or premises, are true. When we are checking whether the premises are true, we are checking whether the argument is sound. A sound argument is an argument that is valid, and all of its premises are true.

A sound argument is the only argument that can give us knowledge of the conclusion. This is due to that if it is valid, then the conclusion must be true if all of the premises are true, and all of the premises are true. Therefore, the conclusion must be true.

Unsound arguments, are when the argument is valid but at least one of the premises is false, an invalid argument, or (if they’ve really messed up) the argument is invalid and at least one of the premises is false. To get knowledge of the conclusion, the argument needs to be sound.

Returning, once again, to the original argument from the first post:

  • All humans will eventually die
  • I am a human
  • Therefore,
  • I’ll eventually die

This argument is sound. It is valid, as established in the post regarding validity, and both premises are true. So, the conclusion that I’ll eventually die must be true.

One must take care where an argument is unsound. Just because it is unsound does not mean the conclusion is false, it merely means we cannot know the conclusion. You can have an invalid argument, or an argument with false premises, or both, with an otherwise true conclusion. To know a conclusion is false, a sound argument needs to be given to demonstrate as such (something we’ll cover later on). Test yourself on the following arguments to see if they’re sound or unsound:

1. Everyone who went to jail committed a crime. Charles Manson went to jail. So, he must have committed a crime.

2. You should eat things that are good for you. Spinach is good for you. Hence, you should eat spinach.

3. Humans are the only species that feel pain. Animals are not human. So, they cannot feel pain.

4. We shouldn’t ever steal. Cheating on taxes is stealing. Thus, we shouldn’t cheat on our taxes.

If you wish to ask any questions, seek clarification, raise some objections, or check how you went on the test questions, please write them in the comments section and I will try respond as soon as I can.

I highly recommend purchasing the book ‘Understanding arguments’ by Walter Sinnott-Armstrong and Robert Fogelin, which is available for purchase in the link below. If you do purchase the book via this link, you are helping support this webpage. Thank you.

Published by

Andrew Tulloch

I have a Bachelor of Arts majoring in Philosophy and Sociology, with a Political Science minor. I also have an honours degree in Philosophy. I am currently studying for my PhD in Philosophy.

2 thoughts on “Sound and unsound arguments”

    1. 2, 3 and 4 I agree. Regarding number 1, is it true that everyone who went to jail committed a crime? Or to phrase the question differently, are any actual or possible cases where someone goes to jail but was innocent?


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