Chapter 3, Tutorial 1
"Ampliative" Reasoning
and Inductive Argument Types

All the deductive logic, at least all that we'll see, is formal. If we understand the right pattern of reasoning, a valid deduction can be easy to make.

If A or B, but not-A, so B.

We don't even need to know what 'A' and 'B' are about.

But one of the lessons of the very first tutorial of chapter one was that sometimes reasoning goes beyond form. Let's remind ourselves:

We should do this again...


















Informal reasoning typically leads to a conclusion by means of some educated guess-work. Maybe we should call this something like detective work. It takes evidence and amplifies it. (Think CSI, if you watch TV.) Let's review what we know from chapters one and two.

Here's the chapter one formulation of the two kinds of reasoning:

Formal reasoning (of deductive logic) manipulates the statements of evidence and performs something of a calculation on them in virtue of their form or sentential structure.

(Think about the simple "process of elimination" argument for Chris's B grade: "Either A or B but not A, so B".)

Informal reasoning (of inductive logic) interprets the evidence to form a conclusion. This thinking amplifies the evidence -- often by generalizing, predicting, or uncovering the best account of this evidence.

(Think about figuring out that Chris is in love. This may be a "best guess" but it may be a reasonable one.)

Here's a review of our main deductive concepts relating to arguments: