There can’t be two things in the same place at the same time, right? Take your phone in one hand and your keys in the other and see if you can get these two objects to occupy the very same bit of space. Try squeezing them together, and I doubt you will get any further than scratching the surface. But some philosophers think that two things can be in just the same place at the same time. Take the parcel of cells that composed you when you were three years old. That parcel of cells no longer composes you; all those particles have now been replaced. So that mass of cells that existed when you were three cannot have been just the same thing as you. Those cells are now scattered to all corners of the planet. But you are not; you are still here. So back when you were three, there were really two objects in the place where you were: you, and the parcel of cells that composed your body. And, for just the same reasons, as you now sit there reading this, there are two things sitting in your chair, not one.

But how can that be? You, and the parcel of cells that compose you, are just as solid and impermeable as your phone and your keys. So how do they occupy the same place at the same time?

Metaphysics is about what there is in the world, and what those things are like. People who do metaphysics are particularly interested in challenging commonsense assumptions about what there is, and what those things are like. On this course, we will be looking at the puzzle introduced above, and many others like it. We will try to work out amongst ourselves whether commonsense can be defended, or whether philosophy should lead us to radically revise our views about the world.