Suppose that I have a yellow, green and purple striped mental image. We may also introduce the philosophical term ‘sense datum’ to cover the case of seeing or seeming to see something yellow, green and purple: we say that we have a yellow, green and purple sense datum. That is I would see or seem to see, for example, a flag or an array of lamps which is green, yellow and purple striped. Suppose also, as seems plausible, that there is nothing yellow, green and purple striped in the brain. Thus it