What is this? Most people see a mass of black blobs and then gradually recognize a photograph of a Dalmatian.

“What is interesting is that the outline shape on the picture surface that is experienced as resembling that of a dog is not seen as an outline shape at all unless the dog is seen in the figure,” writes University of British Columbia philosopher Dominic McIver Lopes. There’s no dog-shaped outline to notice; the contour of the dog’s body is invisible. To see the contour we must first see the dog … but how do we see the dog without the contour?