In message Pine.OSF.3.96.1010516104017.30747A-100000@bscomp, Iain Coleman ijc@bas.ac.uk writes
For example, 'Your observations don't prove that the Earth goes round the Sun, because you're a useless wanker with no more right to live on God's Earth than a weasel' commits the ad hominem fallacy. On the other hand, 'The motion of the Earth is demonstrated quite unambiguously by Foucault's pendulum, you stupid cunt', is a valid argument followed by an insult.
Trust an astrophysicist to pick that example:-) It's interesting to see the Church's argument nowadays as to why it was right to treat dissenters as it did...
I thought he did understand the difference, but was not very good at demonstrating what he meant. The above is a much better demonstration of how to be rude without indulging in the ad hominem fallacy.