Ed T:

Your account of the lecture on Zeno's paradoxes sounded to me like a good example which separates 'serious philosophers' from 'people who attend philosphy lectures'. Zeno's claim (simplifying greatly) was that he had proved movement was an illusion. Now, a serious philospher would either accept the argument as true or false (or, in fact, one of several other possibilities, but let's not go into that now.) Let's assume they think the argument is correct and that movement is illusion. Why on earth should the illusion stop just because you recognised it? Most illusions don't - look at optical illusions for example. The 'illusion' of the professor walking out of the class raises not the slightest complication. What does, though, is why some illusions seem possible, but others - flying unaided through the air for example - appear impossible. Are there different types of illusions? And this is what a serious philosopher would be thinking about for the next few days, weeks, or whatever.

On the other hand, our student could think the argument was false, even self-evidently false. Then they would spend the next few days, weeks or whatever, trying to understand where the flaw in the plausible sounding argument was. (And the chances are, in this day and age, that that is why the department told you about it in the first place, unless it was a history of philosophy lecture.)

Someone who is just attending philosophy lectures, on the other hand, would go to the pub, gym or whatever, and not think about it deeply at all.