 To specify the necessary conditions for the truth of the sentence “that figure is a square” is to specify a number of conditions including “that figure has four sides”, “that figure is on a plane”, and “that figure is closed”. If any one of these latter conditions is false, then the sentence “that figure is a square” is also false. Conversely, the truth of “that figure is a square” is a sufficient condition for the truth of “that figure is closed”. The inferential relations in this case are modelled—even if inadequately—by an operator such as hook.