Impossible objects are two-dimensional line drawings that initially give the clear perception of coherent three-dimensional objects but are physically impossible. Here are some famous examples. The “blivit” looks sensible enough at first glance, but on closer inspection, it becomes clear that such an object cannot exist because the three round prongs on the left end do not match up with the two square ones on the right end. Similarly, the continuous three-dimensional triangle that we initially perceive cannot exist because the surfaces of the locally interpretable sides do not match up properly (Penrose & Penrose, 1958).
One of the most interesting things about impossible objects is how clearly they show that our perceptions are internal constructions of a hypothesized external reality. If visual perception were merely an infallible reflection of the world, a physically impossible object simply could not be perceived. It would be as impossible perceptually as it is physically. Yet people readily perceive such objects when viewing properly constructed images. This fact suggests that perception must be performing an interpretation of visual information in terms of the three-dimensional (3-D) objects in the environment that might have given rise to the images registered by our eyes. Moreover, the kinds of errors that are evident in perceiving impossible objects seem to indicate that at least some visual processes work initially at a local level and only later fit the results into a global framework. These objects actually make good sense locally; it is only in trying to put these local pieces together more globally that the inconsistencies become evident.
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