This fourfold of modal verbs was mentioned in an earlier post A Story for Everyone, but was not shown explicitly. It consists of can, may, must, and should.
[*6.6, *6.36,*6.60, *7.188]
In his Translator’s Introduction to Schopenhauer’s The World as Will and Representation, E. F. J. Payne concisely summarized the Fourfold Root.
Our knowing consciousness…is divisible solely into subject and object. To be object for the subject and to be our representation or mental picture are one and the same. All our representations are objects for the subject, and all objects of the subject are our representations. These stand to one another in a regulated connection which in form is determinable a priori, and by virtue of this connection nothing existing by itself and independent, nothing single and detached, can become an object for us. …The first aspect of this principle is that of becoming, where it appears as the law of causality and is applicable only to changes. Thus if the cause is given, the effect must of necessity follow. The second aspect [knowing] deals with concepts or abstract representations, which are themselves drawn from representations of intuitive perception, and here the principle of sufficient reason states that, if certain premises are given, the conclusion must follow. The third aspect of the principle is concerned with being in space and time, and shows that the existence of one relation inevitably implies the other, thus that the equality of the angles of a triangle necessarily implies the equality of its sides and vice versa. Finally, the fourth aspect [willing] deals with actions, and the principle appears as the law of motivation, which states that a definite course of action inevitably ensues on a given character and motive.