Soundness

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In deductive logic, soundness is a property of arguments importantly related to validity. An argument is valid if and only if it is impossible for the conclusion to be false while the premises are true (i.e. the conclusion must follow from the premises), but an argument is sound if and only if it is valid and its premises are actually true.

Note that an argument can be valid without being sound. Take for example:

P1: If someone knows and employs logic and scientific reasoning, then they do not propagate woo
P2: Deepak Chopra knows and employs logic and scientific reasoning.
C1: Therefore, he does not propagate woo.

The inference from P1 and P2 to C1 is valid (affirming the antecedent) and thus the argument is valid. However, because P2 is false, the argument is not sound.