Now consider a propositional function Fx in which the variable argument x is itself a propositional function.

If Fx is true when, and only when, x is determined to be either 4) or some other propositional function extensionally equivalent to (A, then the proposition F4 is of the form which is ordinarily recognized as being about the class determined by )x taken in extension - that is, the class of entities for which 4)x is a true proposition when x is determined to be any one of them.

One of these is the position of the line MN through the sun at F in which the plane of the orbit cuts some fundamental plane of reference, commonly the ecliptic. This is called the line of nodes, and its position is specified by the angle which it makes with some fixed line FX in the fundamental plane.