In the 1950s, Italian psychologist Gaetano Kanizsa introduced a simple yet captivating visual trick. His namesake illusion—a set of shapes arranged to suggest the presence of an imaginary square—demonstrated how the human brain can fill in gaps to perceive something that isn’t actually there. The square doesn’t exist, but it’s impossible not to see it.

The illusion is so effective that even animals are fooled by it. In one experiment, cats were observed to sit inside the boundaries of the imaginary square created by the shapes. It’s as though they recognized the space as something real, something defined.

At first glance, the Kanizsa illusion seems like a fascinating quirk of visual perception, but its implications go much deeper. It highlights how people—and even other species—respond not just to what is there, but to what is implied.