“Seeing is believing.” So we are often told. You do not, however, have to go far to find instances in which seeing is misleading. For example, look at Figure 4.1 and ask yourself whether the shaded surfaces of the two figures are identical in shape. If you are like most people, you will judge that the shapes differ – one long and narrow, the other closer to square. However, you can convince yourself that they actually have identical dimensions if you rotate one of them 90 degrees and measure the corresponding sides. Your visual system misled you in this case about what actually exists in the world, creating what scientists refer to as an artifact (or artefact, especially in older publications).