The problem of Inverted Qualia is, as Dennett says: “
one of philosophy’s most virulent memes”.
Although many philosophers have espoused variants on it, John Locke’s is perhaps the most popular and enduring example. Put simply it asks: “how can you every be sure that the colour I see is the same colour you see?”. When you see a strawberry as red, I might see it as blue, but by convention and by language, we both call it ‘red’. Most often the problem is phrased using visual examples, but it could be extended to any sensation we have.
Generally, this is a problem in epistemology because it means we can never know what the actual state of affairs are (this belief in ‘actual’ is called ‘realism’). This leads us to scepticism.
Furthermore, we can therefore never rightly attach a truth value to the claim ‘x is red’ without the caveat ‘x only appears red to me’ (this raises all sorts of complicated discussions regarding theories of truth which we cannot explore here). This would then commit us to a Nietzschean perspectivism (tbc soon…)
In short, inverted qualia is a problem of scepticism - where we must admit we cannot know a whole raft of things - namely what is ever going on in someone else’s ‘mind’s eye’ (the experiences they are having - their qualia).
Finally, though, it is a problem too for physicalist theories of mind. These state that we can completely and sufficiently describe all mental events in physical terms. The problem of inverted qualia, though, maintains that if we posited two identical humans, with an identical physical make up, it is conceivable that they could still have different qualia. Therefore, physicalism cannot give a full account of the mind (see also Chalmer’s Zombies).
If nothing else, the problem is a great introduction to philosophy, and is often the first question budding philosophers ask on the road to deeper and thornier issues.