Dice on Steps Optical Illusion

Dave Barlow created this illusion and frames it like this,

"The picture shows exactly the same model with two dice set upon it, from opposite viewpoints. The person with the uppermost view swears that the dark die is higher than the light one. The person with the lower view is certain that his friend is seeing things, because clearly, the lighter die is higher than the dark one.

Who is right?"

Dave Barlow
copyright 2012
used w/permission