Deduction vs. Induction

By Peter Bokulich

What is the difference between deduction and induction? We usually focus on the fact that deductive arguments are rigorously truth-preserving (when they’re valid, that is) while inductive arguments at best make their conclusions more likely.

But there’s another, arguably more important feature that gets little notice.

When you have a deductive argument with uncertain premises, adding more uncertain premises will always lower the likelihood that the conclusion is true. More premises leads to less confidence.

With inductive arguments (broadly conceived) the reverse is true. Including more uncertain premises can raise the likelihood that the conclusion is true.

A deductive argument is like a chain. It is only as strong as its weakest link.

An inductive argument, on the other hand, is like a rope. Each individual thread may be weak, but they are combined in such a way that the whole is far stronger than any of the parts.

The challenge we face is in understanding how we can put together a number of weak claims to produce a structure that can strongly support a conclusion. That’s the real problem of induction.