Dennett references Wilfrid Sellars's distinction between the "manifest image" and the "scientific image", which correspond to the everyday view of the world and the scientific view of the world. Nagel quotes Dennett describing the manifest image as:
full of other people, plants, and animals, furniture and houses and cars…and colors and rainbows and sunsets, and voices and haircuts, and home runs and dollars, and problems and opportunities and mistakes, among many other such things. These are the myriad “things” that are easy for us to recognize, point to, love or hate, and, in many cases, manipulate or even create…. It’s the world according to us.while the scientific image is:
is populated with molecules, atoms, electrons, gravity, quarks, and who knows what else (dark energy, strings? branes?).According to Dennett, the scientific image describes what the world is really like, while the manifest image is just how the world appears to us, a set of "user illusions" evolution has equipped us with to get on with the world. Nagel doesn't mention him but Kant is lurking behind here, as he always is with Dennett.
As is typical with Dennett, what is most important is not what he says but what he leaves out. In his description of the manifest image, in particular, we can include not just homeruns and haircuts, but also telescopes, microscopes, voltmeters, scientific conferences and the scientific method. In other words, it's only through the manifest image that the scientific image is even possible or has meaning. The relationship between them is not that of equals, but of priority: The manifest image is prior to the scientific image both logically and temporally. Thinking you can undermine the manifest image with the scientific image is like thinking you can observe real bacteria with a fake microscope.