An inductive argument does not promise certainty, so we do not call it valid or invalid. We ask how well it supports its conclusion.
Key idea
An inductive argument is strong when its premises, if true, make the conclusion likely, and weak when they do not. A strong argument whose premises are actually true is cogent.
Why it works
Inductive reasoning goes beyond its evidence: the next train has not left yet, however punctual the last forty were. So support comes in degrees, and new facts can always change it. A fact that gives a reason the pattern might break weakens the argument; a fact that rules out such reasons strengthens it.
| Deductive | Inductive | |
|---|---|---|
| Good link | valid | strong |
| Good link + true premises | sound | cogent |
| Can new facts change it? | no | yes |
Worked example: Strong or weak?
"Ben flipped a coin twice and got heads both times, so the coin will probably always land heads."
- Two flips are far too few to show a pattern; a fair coin does this one time in four.
- If the premises are true, the conclusion is still not likely.
Answer: Weak.
Worked example: New facts
"Every time we have eaten at the Elm Diner, the food was good. So it will probably be good tonight." How does each fact change it? (a) The diner got a new cook last week. (b) The diner has blue booths.
- (a) gives a reason the pattern might break: the cook is the source of the good food. It weakens the argument.
- (b) has nothing to do with the food. It makes no difference.
Answer: (a) weakens it; (b) makes no difference.
Common mistake
Calling a strong inductive argument "valid", or thinking a strong argument cannot be weakened. "Valid" is for deduction. And any inductive argument can be strengthened or weakened by more facts; that is part of what makes it inductive.