The term “real” is extremely ambiguous. A trip to an art museum brings out one interpretation. For example, the French Impressionist Edgar Degas (1834–1917) painted ballet dancers practicing in mirrored rooms. Some dancers in Degas’ paintings show dancers in front of, and reflected in a mirror; some show mirror images of dancers off to the side. Which, if any, is a reflection of a real person? Many other artists’ paintings challenge viewers to ponder reality. Sometimes the work shows reflections in windows, outside open windows, or even reproductions within a painting inside the painting.

Our research provides a rather simple way to determine whether an artist assumed that the reflections or reproductions in their paintings were “real” or not. This is to ask observers to count the number of separate objects in a particular painting. If a mirror captures the reflection of an object in front of it, it cannot be counted, but an object shown outside an open window, can. Why? Our implicit counting knowledge prohibits the double counting of an item. So, if you do not count the reflection in the mirror, you do not think it is a real item. To explain, I need to cover a bit about your counting knowledge.

When we count, we must honor the one–one principle and, therefore, uniquely tag every object once and only once. If we double tag an item, we will use too many tags and get the wrong answer to a “how many?” question. All normal adults and many young children are good counters. Even four-year-olds will correct a puppet that counts an item twice. If one counts an exact mirror copy of a foregrounded item, one commits a principled violation. In our experiments with adults we took advantage of this fact.

Dana Chesney and I showed college students line drawings of frames that we dubbed a mirror, picture frame, or an open window. Within each there was a reproduction of the objects drawn in the foreground (e.g., two chairs). So, two chairs were shown in the mirror, two were outside the open window, and two smaller ones were in a painting on the wall. People simply were asked how many were present in each kind.