  The fact that we see these as we do can be explained in Bayesian terms. To figure out the most likely scene/source of (a), the visual system must assign a probability to the hypotheses \(H_2\) (that the circle in a is convex) and to \(H_2\) (that it is concave). One key assumption the visual system makes is that the scene in both (a) and (b) is illuminated by a single light source coming from overhead. So if the bottom of the circle is in shadow, we tend to see it as convex; if the top, we tend to see it as concave. When we look at (a), this assumption about the light source translates into the prior probability of \(H_1\) being higher than the prior probability of \(H_2\). So the priors in this case give us an antecedent ordering of the hypothesis space (here we ignore other hypotheses that could account for the image), and the visual system settles on (a) as convex.