An analogy is defined in the Merriam-Webster online dictionary as:

1. inference that if two or more things agree with one another in some respects they will probably agree in others
2. a: resemblance in some particulars between things otherwise unlike; b: comparison based on such resemblance
3. correspondence between the members of pairs or sets of linguistic forms that serves as a basis for the creation of another form
4. correspondence in function between anatomical parts of different structure and origin

Two important points are that:

• the correspondence may be functional, structural, or otherwise; and
• one can reason by inductive inference about predicted similarities between entities, based on observed similarities.

One may say too that analogies “…constitute an uncommon juxtaposition of the familiar and the unusual.” [Cas07]

Here are some examples of famous discoveries/inventions that resulted from analogical reasoning.

• Lord Kelvin developed the mirror galvanometer from noticing sunlight reflecting from his monocle.
• The engineer Sir Marc Isambard Brunel invented the caisson from observing ships worms tunnelling through wood.
• Benjamin Franklin invented the lightning rod from observing that pointed rods discharged Leyden jars when in proximity to them.
• An anonymous naval officer who invented the take-off ramp on early aircraft carriers after seeing a water-ski jumper (these ramps are still in use for ships supporting the Harrier “jump jets.”)
• The inventor of velcro developed his idea from a direct analogy to plant burrs and dog's fur. Fig. 1: Velcro, close up.
• The coronary stent, a device to reinforce an artery while permitting it to move in a natural way to support the flow of blood, was developed so as to physically resemble scaffolding, from which its inventor drew an analogy early in its development.
• The space coffee cup invented by astronaut Don Pettit works the same as the system that feeds fuel into the rocket engines when operating in zero gravity.
• The Dyson Vacuum may have been invented as a result of analogical reasoning.

Research has also found a strong correlation between analogy and metaphor on the one hand, and design creativity on the other [Cas07]. Metaphors can:

• help to identify and capture design concepts, and define goals and requirements;
• help find unconventional solutions during concept design;
• assist in problem reflection;
• help to break away from the limitations imposed by initial problem constraints;
• explore unfamiliar design alternatives; and
• establish novel associations with the design problem.

Successfully reasoning by analogy requires two kinds of knowledge: a deep knowledge of the design problem, and a deep knowledge of many other kinds of natural and artificial objects from/to which one may analogize.

This implies that it is very important to take the time to study and reason about a design problem before starting to think about solutions. Also, it's very important to have as broad an experience base as possible.

The best designers will constantly revisit the problem as they attempt to solve it, not only for verification purposes, but to reinforce their understanding of the problem to promote analogical thinking. The best designers also tend to be well-travelled and well-read, having worked in many different areas; this broadens their experience base and facilitates analogical reasoning.

The general principle of reasoning by analogy is to study a problem with the goal of looking for similarities between it and other problems you already know how to solve. Don't limit yourself to thinking only about structure, but think also about function (especially), shape, manufacturability, materials, etc.

As you start to think of analogies to the problem, list the solutions to the problems that you've identified. Once you have a few of these, revisit each solution and try to borrow concepts that may apply to the problem you have to solve.

There is a branch of philosophy concerned with this kind of reasoning, called synectics; a fair amount of information on this topic is available through the Web (try Google to find out more).

• analogical reasoning