unsound

Unsound is an adjective used to describe something that lacks solid foundation, reliability, or correctness. It can apply to reasoning, decisions, structures, or practices, and is often contrasted with sound. The term conveys a negative assessment of quality, safety, or validity.

In logic and philosophy, an argument is unsound if it is not sound. Soundness requires that the argument be valid (the conclusion follows from the premises) and that all premises are true. If an argument is valid but one premise is false, it is unsound; if the argument is invalid, it is unsound regardless of premises. For example, an argument such as “All birds can fly; penguins are birds; therefore penguins can fly” is valid but unsound because the premise “All birds can fly” is false in reality.

In construction and engineering, unsound describes structures or components that are not structurally reliable or safe. This can result from material decay, damage, design flaws, or excessive wear. An unsound structure may require restricted access, temporary stabilization, repairs, or demolition to prevent failure and protect occupants.

Other uses of the term appear in legal, financial, and scientific contexts. In law, the phrase unsound mind has historically referred to a lack of mental capacity to make legal decisions. In everyday language, unsound can describe practices, policies, or conclusions deemed unreliable, unsafe, or ethically questionable, such as unsound financial schemes or unsound scientific methods. Across contexts, the term signals a judgment that something should not be trusted or relied upon without correction.