logic
Logic is the study of good reasoning — of how a conclusion is supposed to follow from the things you already accept. Think of an argument as a small machine: you feed in some statements (the premises), and out the other end comes a conclusion. Logic asks one sharp question about that machine: if the inputs were true, would the output have to be true as well?
When the answer is yes, we call the argument valid — the conclusion is locked in by the shape of the reasoning, not by luck. "All cats can fly; Whiskers is a cat; so Whiskers can fly" is a perfectly valid argument. The reasoning is airtight even though the first premise is nonsense. That is the crucial twist most people miss: validity is about the form of the argument, not the truth of what goes in.
So an argument can be valid yet still lead you astray, because one of its premises is false. To actually trust a conclusion you need both: reasoning that holds together and premises that are really true. Logic guards the first half — it keeps the machine honest — while it falls to evidence and observation to supply the second.
Valid but false: "All cats can fly; Whiskers is a cat; therefore Whiskers can fly." Flawless form, false premise.
Validity is about form; soundness also requires the premises to be true.
The word comes from the Greek logos — "word, account, reason." Aristotle was the first to set logic down as a formal discipline in the 4th century BCE; logicians later coined "sound" for the gold standard: an argument that is both valid in form and built on true premises.