 Decidability implies stability, but not conversely. The conjunction of stability and testability is equivalent to decidability. Brouwer himself proved that “absurdity of absurdity of absurdity is equivalent to absurdity” (Brouwer [1923C]), so every formula of the form \(\neg A\) is stable; but in \(\mathbf{H–IPC}\) and \(\mathbf{H–IQC}\) prime formulas and their negations are undecidable, as shown in Section 5.1 below. 