An argument from analogy involves noting the shared properties of two or more things, and inferring that they also share some further property. The form of the argument is as follows:

        P and Q are similar with respect to properties a, b, and c.

        Object P has been observed to have further property x.

        Therefore, Q probably has property x also.

Several factors affect the strength of the argument from analogy:

        1. The relevance of the known similarities to the similarity inferred in the conclusion.

        2. The number and variety of the examples in the analogy.

        3. The number of characteristics shared by the things being compared.

An argument from analogy is weakened if it is inadequate in any of the above respects.

Example:

According to Steven Jay Gould (in his book, Leonardo’s Mountain of Clams & The Diet of Worms), Leonardo da Vinci attempted, but failed, to explain the evident upward movement of water within and on the Earth’s surface (how does rain and snow get into the sky, a spring on a mountain side, and so on) because his theory was that the behavior of the Earth was analogous to the functions of the human body (that is, that the human body was a microcosm of the Earth) and so the Earth’s water was analogous to the body’s blood. However, as Leonardo was evidently aware, for the analogy to be strong there must be a similar mechanism for moving the fluid up against its gravitational tendency to settle to the lower parts, for both the body and the Earth. At that time, various mechanisms had been proposed for this in the case of the blood (note that the theory that the heart circulated the blood was not proposed until a century later), none of which was plausible as an explanation for the upward movement of the Earth’s water. Leonardo never found a mechanism that explained both phenomena convincingly and it is now apparent that it was because his analogy is inadequate to the task. That is, it is a false analogy.

Audio clip: Adobe Flash Player (version 9 or above) is required to play this audio clip. Download the latest version here. You also need to have JavaScript enabled in your browser.