Usually when I address qualia, the way an experience “feels” to us, I focus on demonstrating that they aren’t extra non-physical properties that exist in addition to the physical ones. This is because to explain qualia is considered a serious challenge for a materialist theory of the mind, the theory I find most plausible overall. And because I am an internalist, generally, when it comes to the mind my explanations of qualia are internalist as well, meaning that they identify qualia with some features of how the physical implementation of the mind, the brain, operates. But materialists are not all internalists, although I think they should be. Some give externalist explanations for qualia, identifying qualia with some features of objects in the world, or with a combination of object features and brain features. Generally this is done in two ways, either through a strong kind of direct realism, where the qualia of an experience, say the red of the visual experience of an apple, is identified with the actual red color of the apple, or through an externalist theory of representation, where brain states represent the world, but the content, and qualia, of those internal states is provided by the properties of the objects themselves.
This account may seem plausible to some, especially if you already have externalist leanings, but it simply doesn’t fit the facts about experience and qualia. To support this claim allow me to provide two facts about experience that are difficult to explain in an externalist framework. The first is the fact that we see some colors that don’t actually exist. Of course most colors do exist, red is a certain wavelength of reflected light, as is green and blue. So for these colors the externalist account of qualia might seem to make sense, meaning that when it feels like I am seeing red maybe what I am really feeling is the wavelength of the light. But some colors, like pink, don’t exist. You see, the human experience of color maps color to a kind of wheel, with three primary colors from which all other colors can be derived by combinations. But real color does not work this way; the wavelengths of light are linear. The color wheel exists because when our eyes receive two different wavelengths of light from the opposite ends of the visual spectrum, red and violet, it invents a new color, magenta, instead of averaging them, which would yield green. (You can read a more detailed explanation of this phenomena here) If the externalist account of qualia was right then when we saw magenta it shouldn’t feel like magenta, it should feel like both violet and red, which is what it actually is.
A second problem with the externalist account of qualia is that it can’t explain synesthesia. Synesthesia is a name for a mental “quirk” that some people have (it’s not a disorder, since it doesn’t impair them in any way, nor does it interfere with their life, it simply is difference in the way they experience the world). People with synesthesia experience some kinds of objects of experience as not only “feeling” like we would expect them too, but also “feeling” like some other, unrelated, kind of object of experience. For example, some people with synesthesia experience numbers or sounds as “feeling” like certain colors, meaning that for them these experiences have the qualia of, say, a certain note, and the qualia of a certain color. And the mappings of one kind of qualia to the experience of another sort is constant for a person with synesthesia, generally (they won’t experience a single note as green one day and blue the next), but they do vary from person to person with synesthesia. Again, the externalist account cannot for synesthesia. Since the music doesn’t produce light it shouldn’t feel like a color to these people, meaning that the qualia of feeling like a certain color is not identical to certain external properties concerning light. In fact it isn’t identical to certain external properties at all, synesthesia seems to preclude that possibility.
Can an externalist account of qualia somehow explain these oddities? Possibly. But the simplest explanation of them is simply that the qualia of an experience depends only on internal features, and that it is only prompted or caused by certain external features in a normal experiencer, not dependant on them. And since this is the simplest and most direct explanation it seems reasonable to accept it as the better one, assuming that a satisfactory internalist explanation of qualia can be provided.