…But the problem with sense data is that they don’t exist. Here are a couple of reasons for being suspicious of them. First, there isn’t any empirical evidence of them whatsoever, apart from our thinking that they exist. (This is the downside of having your esse be percipi.) There’s no scientific instrument that can register the presence or absence of a sensation. We can measure the electrical activity of somebody’s brain, of course, and we can watch their pupils dilate, but the only way to find out whether they are experiencing anything is to ask them, and take their word for it. This is the so-called “hard” problem of consciousness: namely, to explain why there should be any consciousness, when nothing we can see “from the outside” gives us any information about it, or indeed any evidence of its existence.

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