non-constructive methods

In mathematics and the theory of computation, a proof is non constructive if it enables you to know that some construction exists, but does not enable you to identify it. For example, it might be known that a solution to a problem, or a method for winning at a game, exists, without it being thereby established what the solution or method is. Non-constructive proofs are the particular target of intuitionist criticisms of classical mathematics and logic.