Seeing a rose seems to give me information about a flower out there in the world. But smelling it, blindfold, is a curiously internal experience: I can suppose, even confidently, that it’s a rose I’m smelling, but this feels like a surmise, and one based only on an impression in my mind. The visual world seems to be made up of independent objects with observable properties, but the world of smell seems to exist only in our consciousness.

If there were no creatures here to observe them, roses would still be red. But would they still smell sweet?