Unsound

Tom's argument was unsound.

Your position is unprovable and unsound.

The building is structurally unsound.

unsound mind

In the ensuing financial panic, many persons sustained heavy losses, and large numbers of unsound schemes succumbed.

A striking example comes to mind, in which a scheme to improve the existing buildings finished up as virtually a complete reconstruction, owing to the unsound condition of the original structure!

While he is reasonably effusive about inter-city travel, he is heavily disparaging of all types of stopping service, including those on otherwise busy main lines. His analysis is not entirely unsound, and he tackles some of the questions head on.

... A horse labouring under a temporary injury, capable of being speedily cured, is not unsound within the meaning of a warranty of soundness ...

You would not like Nietzsche, sir. He is fundamentally unsound.