I haven’t read too many philosophical works in the form of dialogues, but what I have seemed to tend too much towards “everyone nods as one character explains the author’s views.” More realistically, it might go something like this…
Protagonius: So, my dear friend Simplexitus, we are here to discuss our theories of the shape of the Earth.
Simplexitus: Yes, or your theory, anyway. I know that you say that the Earth is round, though so large we do not notice the curve.
Protagonius: Yes. This is based on scientific observations.
Simplexitus: That may be so, but it cannot be correct. Your view has absurd consequences.
Protagonius: What would those be, Simplexitus?
Simplexitus: People on the bottom side of the Earth would fall off.
Protagonius: But, my dear Simplexitus, you are aware of the idea of gravity, are you not?
Simplexitus: Yes, though I do not agree with it. So I take it you are saying it is the gravity of Earth that keeps people on the bottom side hanging on?
Protagonius: Yes, and on the “top” side as well.
Simplexitus: That seems redundant. But you are saying people on the bottom half would be held from falling as if by magnets?
Protagonius: No, they would stand on the ground just like on every other side. Gravity would pull them towards the centre of the Earth, and that’s why anything falls. Because of gravity.
Simplexitus: Surely you are confused. Gravity or not, they would fall down if unsupported from below.
Protagonius: Why is that, Simplexitus?
Simplexitus: Because there would be nothing underneath them, so they would fall down.
Protagonius: That is exactly what is explained by gravity, Simplexitus. But gravity makes things fall towards the centre of the Earth. It’s a scientifically established principle.
Simplexitus: Let us try a thought experiment. Suppose that you remove this round Earth of yours, leaving everyone standing on nothing.
Protagonius: Very well.
Simplexitus: With nothing to keep them from falling, would they all not fall in one direction — down?
Protagonius: No, because there would be nothing for them to fall down to.
Simplexitus: You are letting your theoretical conceptions cloud your judgement, Protagonius. Down is a direction, not a thing. But imagine this instead: the Earth was still there, but its gravity was extinguished. Now, obviously those people standing on the bottom and side would fall off, whereas those on the top could continue as it was. This shows that your gravity is both superfluous and absurd.
Protagonius: It shows nothing of the sort. What would happen in both cases is that people would float because they would have nothing to fall towards — maybe the Moon, but let’s ignore that. Down is relative to the centre of the mass you are currently standing on.
Simplexitus: Really, Protagonius? Forget your theoretical preconceptions and engage your intuition. Do you not see things happening as I described them?
Protagonius: Well… perhaps I could see that, yes. But that is only because my intuitions are not as reliable as my wider understanding of how the world actually works.
Simplexitus: But your wider understanding presupposes the theory you are trying to prove. On the other hand, my intuitions are pure and original. I did not come up with a theory first and only then consult my intuitions.
Protagonius: My position is not based on intuition, it is based on empirical observations.
Simplexitus: My position is supported by much more direct empirical observations, whereas yours can be shown to be faulty by reasoning and thought experiments.
Protagonius: Consider this, then. If the Earth is imperceptibly round, then what you experience as things falling down is them moving towards the centre of the Earth, correct?
Simplexitus: Given your assumptions, yes.
Protagonius: And in that case, these falling events are entirely explained by the power of gravity to draw things towards large centres of mass, correct?
Simplexitus: In a purely mechanical sense, yes.
Protagonius: So we can agree that what actually happens is explained by my model?
Simplexitus: No, Protagonius. I can grant that, in a purely mechanical sense, your model makes happen the things I have observed myself. But it does not capture the primitive fact that being unsupported makes things fall down. If it were true, there would be no one direction in which everything falls.
Protagonius: As I have told you, dear Simplexitus, empirical evidence shows that there is not. People do not fall off the opposite side of the Earth.
Simplexitus: That is only assuming that there is an opposite side to the Earth. All of your arguments presuppose what you are trying to prove.
Protagonius: No, it is your arguments that presuppose your position. Mine is based on empirical observations, but yours is built on the presupposition that there is a single direction that is “down” and that things will always fall in that direction if unsupported. You take this as a premiss you do not articulate or argue for.
Simplexitus: I hold these things to be self-evident, even analytical. How would you define a direction? Would you not agree that what we normally mean by a direction is one of the two ways on which you can follow a straight line of infinite length?
Protagonius: Yes, I would agree to that.
Simplexitus: And is “down” not a direction?
Protagonius: Given what you just said and I agreed to, I would have to disagree. It is not on one line, it is towards the centre of gravity and so different directions from different places.
Simplexitus: Really, Protagonius? I wonder at your commitment to your theory. Is down not one of the prototypical directions? Would not the ordinary man, uncorrupted by philosophy, definitely agree that it is a direction?
Protagonius: Indeed he might, Simplexitus, but we have progressed beyond that and defined the term more precisely. You have said a direction is something that is always the same, no matter where you are. Down is not like this.
Simplexitus: This does not make sense if we are to really discuss the direction “down” properly. Setting aside that it is clear what direction is down, we might just postulate it. It is this direction that I am pointing to now.
Protagonius: But then, there is no reason everything would always fall in that direction.
Simplexitus: If I pick up this rock and drop it, it will fall in that direction. We both know it without even having to test it.
Protagonius: Certainly the rock would fall in that direction here and now, in these circumstances. It would not do so on the other side of the Earth.
Simplexitus: But our intuitions are not tied to the particular circumstances. Certainly you may imagine rocks falling up, like you might imagine a city in the clouds, but you know it is a fantastical image. Remember my thought experiments.
Protagonius: It would not look like a rock falling up if we were on the other side of the Earth.
Simplexitus: But that is only how it would look. Our intuitions might even be fooled, certainly, if it looked exactly as though it was falling down. However, if we take a broader view of the Earth as a whole, you will see that only things on one side of it — only a small slice of that side, too — are falling down if they are falling towards the centre. Though, of course, you cannot really “fall” anywhere but down. Obviously this is a part of the definition of falling. In the other cases, you would be flying.
Protagonius: I think, my friend Simplexitus, that we had best stop this dialogue now, for I am feeling a regrettable yet increasing urge to punch you.
Simplexitus: Very well, my friend. Reflect on these truths and perhaps you will yet see reason.