Sound (logic)
Etymology
From Old English "gesund" (healthy, whole, uninjured), of Germanic origin — akin to German "gesund," the same "sound" as in "safe and sound" and "of sound mind." A sound argument is a healthy, whole one.
Sound meaning noise is unrelated — from Latin "sonus."
Sound meaning a strait or channel is a separate origin again — from Old Norse "sund," a strait or a swimming.
adjective · /SOWND/
Of an argument, both valid in form and true in all its premises.
What it names
An argument you actually have to accept — correct structure plus true starting points, the full bar that mere validity doesn’t clear.
Example
The proof was valid but not sound — flawless steps built on a measurement that turned out to be wrong.
Related idea
Its partner is valid: validity checks only the logic connecting premises to conclusion; soundness adds the demand that those premises be true. Every sound argument is valid, but most valid arguments are not sound.
noun · /SOWND/
Vibrations traveling through air or another medium that the ear detects; what one hears.
What it names
audible vibration
Example
The sound of rain filled the room.
Related idea
The everyday meaning, entirely unrelated to the logical one.
adjective · /SOWND/
In good condition; healthy, solid, or reliable.
What it names
the state of being undamaged or dependable
Example
The old timbers are still sound; she gave sound advice.
Related idea
Like a sound argument, something in sound condition is free of hidden flaws — but here the soundness is physical: a building, a night’s sleep, or a body in good working order rather than a valid chain of reasoning.
noun · /SOWND/
A narrow stretch of water connecting two larger bodies.
What it names
a sea strait or inlet
Example
They sailed through Puget Sound.
Related idea
Unlike a fjord, a glacier-carved inlet, or an estuary, where a river meets the sea, a sound is simply a wide channel linking two open bodies of water — and shares nothing but spelling with the sounds you hear.