We know things fall down because gravity pulls them down towards the centre of the Earth. Gravity is a force, or some kind of spacetime curvature that acts like it’s a force.
Suppose you’d never heard of this. And suppose, while you’re at it, that you didn’t know the world was round. Then, quite possibly, you would think it impossible that the world would be round — because the people at the bottom would fall off.
Now, since you know about gravity, you could ask, why would they? In fact, there’s no “down” in space, only somewhere like the surface of a planet, and then “down” is towards the centre. Knowing this, we can ask, why would someone fall off the “bottom” of the planet? Why assume that?
But if you never had heard of gravity and never thought about such things, it might seem obvious to you. Things at the bottom end of the planet would fall off. You’d imagine something being placed there and automatically imagine it falling off. It’d be obvious, not needing an explanation. If someone told you that wouldn’t happen without a reason, you would not, in this scenario, understand what it meant. You wouldn’t think it needed a force. You’d just think it was obvious.
Yet, of course, it wouldn’t be. Things wouldn’t just fall if there wasn’t a force.
We’re lucky to know about gravity, because it gives us such a good example. There are a lot of other questions where people consider something obvious and not needing an explanation, but really it does. There’s some extra premise, like gravity. For example, why do we often automatically assume that if someone does something bad, they deserve to be punished? That’s the kind of thing where people won’t even notice there’s a missing premiss. They’ll just jump from guilt to deserving punishment without asking why that leap should be made. Another example: how many people ask the question of why things farther away look smaller?
We don’t always need to question everything we take for granted. But sometimes we do. When that time comes… well, it’s hard to notice. But remember the example of things supposedly not needing a reason to fall down.