Imagine an alien comes to Earth and demands that you explain how a toilet works. That’s easy for you, right? You’ve presumably been using a toilet your entire life, as has everyone you know. In fact, you may even have encountered an array of different types of toilets, from the old-style one with a pull flush in your grandparents’ house to more modern ones with heated seats in places like Japan. Explaining toilets to aliens should be a piece of cake. You assure your new friend that very soon, he’ll be an expert on toilets.
And yet, as the alien takes a seat to listen, you realize you can tell him about the button or the lever you press to flush the toilet, but you can’t explain much about what’s going on inside the toilet. How does the dirty water leave and the clean stuff arrive? What mechanisms or forces are working behind the scenes? And what about the little thing that rises and falls inside the… what’s it called… the bit at the back…
Perhaps you can partially answer one or two of these specific questions (or even more if you happen to be a plumber), but surely the alien will have even more questions you can’t answer. To think that a toilet is such a simple, everyday item, and yet you actually know much less than you’d predicted. This puzzles you greatly.
Your trouble explaining a toilet to the alien is because of the illusion of explanatory depth: sometimes, having to explain your knowledge brings you to realize how limited it is in reality. You could replace the example of the toilet with many other everyday items, such as locks, car engines, or light bulbs.
The illusion has two parts. First, the “explanatory” part refers to our belief that we can provide a clear, detailed explanation of how something works. The “depth” part reflects our assumption that the explanation will be thorough and complex enough to convey what we’re trying to explain. In reality, however, when we try to dig into the details, we often find that our explanatory knowledge is much shallower than we initially thought. What we actually end up with is the reality of explanatory shallowness.
Both parts are important to the illusion. That is, it’s only when we explain something that the gaps in our knowledge become apparent to us. Before that, we might still believe in our minds that we possess a depth of knowledge. The illusion of the explanatory gap is something that we’re subject to from an early age, with studies showing that children as young as kindergarten overestimate their explanatory knowledge.7
The illusion is far stronger for explanatory knowledge than other domains of knowledge, such as facts, procedures, or narratives.11 And it doesn’t apply if you know nothing about a topic and are happy to admit it. It only works when we inaccurately overestimate our understanding or knowledge of a certain concept or topic.