 He catalogued a number of optical illusions as well (Optics III.7), examining such problems as the way the moon when low on the horizon appears larger than it does when higher in the sky, and the way that when one is in a boat floating down a river, the trees on the shore appear to be moving. Though Al-Haytham was not a skeptic himself, these illusory experiences provided fertile material for later thinkers to consider. Tachau (1988) discusses his wide influence on the scholastic tradition.