What must be true for you to see a yellow station wagon? To begin with, there must actually be a yellow station wagon. If there isn't, you may think – you may even say, quite sincerely – that you see a yellow station wagon, but you are simply wrong. It only looks like a yellow station wagon. Maybe you are hallucinating – or just terribly confused.

Since there obviously are (or were) yellow station wagons I have never seen, what else must be true for me to see one? Once again, most folks will agree that there must be some causal relation between the station wagon and me. It must, somehow, affect me. If the car is in your garage, then I can't see it, because the garage walls prevent a causal interaction. Light reflected from the car never reaches my eyes. If you run the engine, I might hear it (in this case, the causal relation between the car, or the car's engine, and me is via my ears), but I won't see it.

Just affecting me, though, is surely not enough. Many things affect me that I never perceive. I don't have to see or feel poison ivy (the plant) for it to affect me. I know it was there, in the field I walked through yesterday, but not because I saw it. I know it was there because today I perceive the effects – an ugly rash – that it had on me.