This is the reasoning behind mathematics, formal logic, and legal arguments. When a lawyer argues a law applies to a case, they use deduction: the law says X, this situation is covered by X, therefore the law applies. The certainty is real — but it depends entirely on true premises.

Garbage in, garbage out: a valid deduction from a false premise gives a valid-looking wrong answer, and a surprising number of real-world errors trace back to an unexamined premise. "All experts agree" is rarely true. So is "the market always recovers." Before trusting any chain of reasoning, stress-test the premises — a conclusion is never stronger than its weakest one.