Mental events have a certain "subjective" quality to them, whereas physical ones seem not to. So, for example, one may ask what a burned finger feels like, or what the blueness of the sky looks like, or what nice music sounds like. Philosophers of mind call the subjective aspects of mental events "qualia." There is something "that it's like" to feel pain, to see a familiar shade of blue, and so on. There are "qualia" involved in these mental events. And the claim is that qualia cannot be reduced to anything physical.