Defining the Problem, characterizes problem solving as a relentlessly logical process for discovering and displaying the underlying structures that give rise to events we consider undesirable. Our theory has been that the solution to a problem will always lie in tinkering with the underlying structure, as indeed it will if the problem is that we do not like the result the structure is yielding.

However, there is another kind of problem situation where the problem is not that you don't like the result, but rather that you can't explain it. You can't explain it for one of three reasons:

- Because the structure does not exist-as when you are trying to invent something new (e.g., the telephone, underwater tunneling)

- Because the structure is invisible-as in the brain or DNA, so that you have only the results of the structure to analyze

- Because the structure fails to explain the result-as when Aristotle's definition of force did not explain the momentum of a cannonball, or when tools rust mysteriously no matter what you do to guard against it.

It is possible that you may confront one of these structureless situations in the course of an ordinary problem-solving assignment. Although such situations require a higher level of visual thinking than we have been discussing, you will be pleased to know that the reasoning process employed is very similar.

What is required is simply another form of Abduction-a name coined by Charles Sanders Peirce in 1890 to describe the process of problem solving. In calling it Abduction he hoped to emphasize the affinity of problem-solving thinking with Deduction and Induction. Let me explain the difference between the two forms of Abduction, and show you how to use the second.