There are two main ways to explain something. Comparing and contrasting. You compare to something that you do know and fill in the differences, or contrast it to something that you do know and fill in the differences. An analogy is a mechanism for comparing to something that you do know. It comes from the Greek word analogia, which means “similar too”. This logical fallacy corrupts the useful tool of the analogy and extends it down three possible faulty paths.
The left fork in the analogy path is taking the analogy too far. The right is to mistake the analogy for the event. The middle path leads to analogies that have nothing to do with the thing you are trying to discuss, primarily due to ignorance.
Taking the Analogy Too Far: This is where an analogy begins as a useful tool to understand a concept or idea that is unfamiliar to you by placing it in a familiar setting. This initially boosts the speed at which the idea can be absorbed. A good example would be to compare gravity to a distortion in a rubber sheet. This quickly gives a two dimensional sheet a third dimension that changes the path of a marble that is rolling along the sheet and deviates into an orbit around a large body (like a bowling ball). Very quickly do you see how a similar distortion in space caused by our star would bend an object travelling through the solar system and have it end up in an orbit. Taking the analogy too far is to ask what happens when the object is so massive that it rips a hole in the rubber sheet, or placing all of the objects in our solar system in this model and watching them all sink into the central sun – which clearly doesn’t happen.
Mistaking the Analogy for the Event: The analogy is chosen for a conceptual similarity to the process you are trying to describe. Take the rubber sheet above. It has a similar concept of distortion of a surface (space). The error here lies in thinking that space is a rubber sheet and giving space the properties of that rubber, that large objects on the sheet of space are protruding into some other space and so forth. The concept that was analogous was the way objects in motion are distorted because of the stretching of space – the rest is not the same.
False Analogy: Trying to create an analogy that is not actually similar to the process you are describing, because you don’t understand the process you are describing. A common one is to suggest that as far as evolution is concerned, an organism evolving into an human by chance is the same as tornado ripping through a junk yard and making a 747. The faulty understanding is to consider that evolution is complete chance, rather than an evolution of working features that are mutated by chance, where the working features continue and the faulty ones fail, and where the definition of “working” isn’t the same as “wanted”. A better analogy would be to describe how water carves out a gorge from a mountain. The path is not predetermined, however strengths and weaknesses in the rock will resist or succumb to the water, creating a beautiful work of art without the requirement for an artist.
Remember that the correct use of an analogy is a powerful teaching tool. The misuse of the tool can create problems.