In very general terms, three types of logical reasoning:

- Deductive reasoning
- Inductive reasoning
- Retroductive inference

Deductive reasoning is how we arrive at the “truth” – whatever that may be. It means that if A = B and B = C, then A = C. There is often something sequential about deductive reasoning, which I’ll get to a little later on in the chapter.

Inductive reasoning isn’t necessarily true, but suggests the *probability* of something being true. It helps us make a judgment based on past observations: “Joe has been driving for 40 years. He has never had an accident and only one ticket. Therefore, Joe must be a good driver.” What we don’t know is how much driving Joe actually does. Maybe he walks or bikes most of the time. But the probability is high that Joe really *is* a good driver.

Retroductive inference is all about learning things in one situation and then applying these things in a new, but similar situation. Like understanding how to get around an unfamiliar airport: the plane is at the gate. The gate has a number and possibly a letter. Signs will point the way.

All three of these ways of thinking about “stuff” affect our perceptions of “usability.” Remember, when I say “stuff” I basically mean everything – physical objects, interactive objects, services, etc. Anyway, I hope you’ll find this knowledge as useful as I have over the years.

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