You guys have started talking about some stuff that really interests me. Or at least talking about it in a way that interests me. So I'm coming out of my cave to drop some knowledge and take part
.
Incorrect. I think that this is referring to validity, not soundness. Further, it's applying 'truth' in a way that it really shouldn't be applied. The argument can never be true; only the statements (assumptions/conclusions here) can be true. Anyway, to the point.
Validity = if all premises are true, then the conclusion is true.
Soundness = Valid argument where all premises are true.
So if one of the assumptions is incorrect, then the argument is by definition not sound. It can still be a valid argument, however. That much is true. Example would be like... "Things that appear larger than other things are larger than those things. The moon appears larger than Jupiter. Therefore, the moon is larger than Jupiter." That is a valid argument, because if those premises are true, then so is the conclusion. But it isn't a sound argument, because neither of the premises are correct.
Anyway, I think it's really cool that you're using logic to clarify and express your arguments! I'd really like to see more stuff like this in the future, since it will (maybe) get rid of a lot of the back and forth he-said she-said nonsense that forum arguments devolve into. Really nice to see this sort of thing when trying to make your case. I applaud you!