The second Gestalt Principle of perception is Reification. The word refers broadly to making something that isn't real in the physical world, real in the mind. It refers to the mind's ability to "construct" what it needs, especially in spatial terms in Gestalt psyhology, even when incomplete. For example in the example below, the image represents literally three segmented "pie-charts" with their missing arcs facing inward. You might like to think of it, as I know I do, as "a meeting of Pac-men". But we are irresistably drawn to fabricating a triangle which is overlaid upon three circles.
We add depth where there is no depth and find boundaries we are happy to imagine. The reason is something we will encounter again in a number of the upcoming Gestalt Laws, most notably in Prägnanz. Essentially we make things simpler for ourselves and imagine "primitive" shapes like circles and triangles to avoid having to deal with more complex realities. It has been hypothesised this may have to do with making our cognitions more efficient and preserving our mental faculties, or an evolutionary process like recognising familiar objects even when partially occluded; like a lion in the grass.