First consider our senses of sight, hearing, smell, taste, and touch, which are major sources of information about the world. Much is known about the physical basis of how these senses work, because they can be studied in nonhuman animals whose senses seem to operate much as our own do. Here is what happens when you see a tree. Light reflects off the tree and into your eyes, where photons stimulate some of the millions of nerve cells in the retina at the back of your eyeball. These cells then send signals along your optic nerve to the back of your brain to the occipital lobe, which begins a complex process of interpreting the retinal input using a series of regions that include parts of the temporal lobe (see figure 3.2). Eventually, the result is a pattern of activation of neurons in the several regions that reactivates an approximation to the pattern of neural firing that constitutes your concept of a tree, allowing you to identify the observed object as a tree.