The following relation between operations F and G is called an adjoint situation or adjunction, and such a relation occurs almost everywhere in logic and mathematics; F(C)→B ⇔ C→G(B). The relation between A∧ and A⊃ which are primitive connectives in logic is a well-known example. So to understand the primitiveness of these connectives, it is necessary to clarify the essence of adjointness. For this purpose the author thinks tentatively that two operations G and F in an adjunction ultimately correspond to two mental functions (i.e. projection and reflexion) which are always found in a primitive state of our consciousness.
