Visual illusions are non-negotiable. Even though one part of the brain knows the images are the same, the perceptual system refuses to submit to that knowledge. Similarly, our decisions can be the product of two sometimes antagonistic systems, sometimes called the automatic and reflective systems.

The Leaning Tower illusion is a product of the assumptions the visual system makes about perspective. When parallel lines, such as those of railroad tracks, are projected onto your retina, they converge as they recede into the distance (because the angle between the two rails progressively decreases). It is because your brain has learned to use this convergence to make inferences about distance that one can create perspective by simply drawing two converging lines on a piece of paper. The picture in the illusion was taken from the perspective of the bottom of the building, and since the lines of the tower do not converge in the distance (height in this case), the brain interprets this as meaning that the towers are not parallel, and creates the illusion of divergence.

The Learning Tower illusion was first described in: Kingdom, F. A., Yoonessi, A., & Gheorghiu, E. (2007). The leaning tower illusion: A new illusion of perspective. Perception, 36, 475–477.

Thank you to Tony Sable for the Leaning Tower of Pisa image above, and to Fred Kingdom for image permission in Brain Bugs.