How many planets are there?

This post at the ever-stimulating Atlas(t) prompts the question: how many planets are there?

An easy question, right? After all, I think we should be able to tell if we bump into a planet! They're pretty big.

So is Pluto one? A planet that is. This illustrates an interesting question because while it might appear straightforward (Pluto either is or isn't a planet) it shows that what we know is actually dependent on our categories for things. Kant said this way back. He said that while we have no direct knowledge of the real world, what we do have is knowledge of our categories for things (try thinking without any categories--it's not possible).

So "planet" is not a natural object, it's a human category. And as the category changes or is challenged, some things may no longer belong to it. Mapping of course is another activity where we don't just take the world, but put it into categories. The trick is to know what categories are operating at any given time.

Bottom line: we may have just lost a planet!