 The notion of “negation as impossibility” is known as “strong negation”. One speaks of the “weak negation” of \(A\) to express that so far no proof of \(A\) has been found. This excludes neither finding a proof of \(A\) nor finding a proof of \(\neg A\) later. Clearly, then, to assert the weak negation of \(A\) is not to assign a truth value besides true and false to it; Barzin and Errera’s claim (see  section 4.3  below) that its treatment of negation turns Brouwer’s logic into a three-valued one is groundless. The distinction between weak and strong negation is important for the so-called “weak counterexamples”.