Deduction
Etymology
From Latin "deductio" (a leading down or away), from "deducere" — "de-" (down, from) plus "ducere" (to lead). Where induction leads the mind in from instances, deduction leads it down from a general rule to the particular case it must cover.
noun · /dih-DUK-shun/
Reasoning in which a conclusion follows necessarily from general premises.
What it names
Use it for airtight inference — if the premises are true, the conclusion cannot be false; the certainty comes at the price of telling you nothing the premises didn’t already contain.
Example
All humans are mortal, Socrates is human, therefore Socrates is mortal — deduction, with no room for the conclusion to escape.
Related idea
Its counterpart is induction, which generalizes beyond the evidence and can be wrong; deduction never adds information, only unpacks it. Sherlock Holmes famously "deduces," but much of what he does — inferring the best explanation from clues — is really abduction.
noun · /dih-DUK-shun/
An amount subtracted, especially from income or a total.
What it names
a sum taken away
Example
After tax deductions, her take-home pay was lower than expected.
Related idea
Both senses ‘take away’ — one a step from premises, the other money from a total. Compare take-home-pay, expense.