Recipe

Evaluate an argument with three questions

Use when: someone offers you an argument and you need to decide whether it gives you a good reason to accept its conclusion

Goal: a verdict of sound or unsound (deductive) or cogent or uncogent (inductive), with the reason for it and a precise statement of what it shows about the conclusion

Taught in Three questions for any argument

1. 1

   Put the argument in standard form, supplying any missing premise charitably. Decide whether it claims a guarantee or a likelihood.

   Example

   "You should get the flu shot, since it cuts your risk of flu a lot." Standard form: (1) The flu shot greatly lowers your risk of getting the flu. (2) [Supplied] Cutting a real health risk at small cost is worth doing. So you should get the shot. The supplied premise bridges the gap from "lowers your risk" to "you should." The shot can't guarantee you stay well, so the charitable reading is a likelihood claim: inductive.

   • it claims a guarantee — it's deductive; use valid/sound below.
   • it claims only a likelihood — it's inductive; use strong/cogent below.

   Why: you can't grade a link until you know which promise it made.

2. 2

   Check each premise against evidence. Mark it true, false, or not yet known.

   Example

   Health agencies' published data support premise 1: true. Premise 2 is a value most people share, and the shot's cost is small: true for most readers.

   Why: this half of the verdict is about the world, and only evidence can settle it.

3. 3

   Suppose every premise is true, even one you just marked false. For a deductive argument, search for a counterexample. For an inductive one, ask how probable the premises make the conclusion.

   Example

   If the shot greatly lowers flu risk and a cheap cut in risk is worth making, getting it is probably the right call for most people: strong.

   Why: the link is the same whether the premises are true or not, so it must be judged on its own.

4. 4

   Sound or cogent only if both checks pass. Otherwise unsound or uncogent. If a premise is unknown and the link is good, give a verdict with an "if" attached.

   Example

   True premise, strong link: cogent. It's reasonable to get the flu shot.

   • both pass — sound (conclusion true) or cogent (conclusion probably true).
   • either fails — unsound or uncogent; the argument doesn't establish its conclusion, which may still be true.

   Why: the verdict is nothing more than the two answers put together, so it can't be reached by skipping either one.