Experiments to probe the nature of one mode of thinking, imagined spatial operations, have been devised. Results confirm that the mind can model physical processes, subjecting them to the geometric constraints that hold in the external world. The experimental work and implications are discussed in detail. (MNS)

Various theories are introduced to explain illusions in which the length, orientation, curvature or direction is wrongly perceived. The role of eye movements in the process of diminution is discussed. (SA)