Charles asked:

My logic professor says that our logic textbook is wrong when it says that invalid arguments are also unsound. My logic professor says that it is a category mistake to call invalid arguments unsound. Instead, he says that invalid arguments are neither sound nor unsound, since the terms ‘sound’ and ‘unsound’ are only used to describe arguments that have already cleared the validity hurdle, so to speak. (Sound and unsound are species of the genus valid, he said, so there are three types of deductive arguments: invalid, valid and sound, and valid but unsound.)

Who should I trust, my logic textbook or my logic professor?

Answer by Craig Skinner

Truly a storm in a teacup, the sort of nit-picking that can give philosophy a bad name.

‘A sound argument is defined as a valid argument which has true premises’ (Guttenplan: The Languages of Logic, 2nd edition, p 26, Blackwell, 1997). All agree about this.

We could therefore say that all other arguments are not sound ( are unsound), including invalid ones. This is the stance of your logic textbook, also of the online Internet Encyclopaedia of Philosophy which says:

‘A deductive argument is sound if and only if it is both valid and all of its premises are actually true. Otherwise, an argument is unsound’.

Alternatively, we could hold, with your professor, that soundness/unsoundness apply only to valid arguments.

It’s not a question of right or wrong, but of convention as to how we use a word. It makes little difference one way or another – there are still three types of deductive arguments: invalid (and unsound); valid and sound; valid and unsound. None of the textbooks on my shelf (Hodges; Newton-Smith; Guttenplan; Priest; plus two advanced texts) takes a stance on this (trivial) matter. Which textbook are you referring to in your question?


Answer by Shaun Williamson

I don’t think it really matters. Although your professor has a point. An invalid argument isn’t really an argument. So it seems strange to say that it is sound or unsound. When you are going to accept or reject arguments you start with valid or invalid then you can move onto sound or unsound as a further classification of the valid ones.

However don’t trust me either, what do you think?