In this paper we will give our reasons for believing that certain current attempts to explain perceptual phenomena on a lower level in terms of known sensorymechanisms are untenable. We will do this by focussing on two topics, lightness perception and the perception of apparent motion. We will summarize some older data (not all of which are sufficiently known) and will describe some recent work of our own. Finally, on a more positive note, we will try to indicate the direction that a theory must take if it is to deal effectively with these phenomena.