Each and every type of "ought", be it religious, moral or legal, must presuppose a certain point of view, a point of view which is constituted by the basic norm of the relevant normative system.
There are many difficulties with this, not least of which is the fact that if we are willing to tolerate the basic norm as a solution it is not clear why we thought there was a problem in the first place.
Since the actual, legal, chain of validity comes to an end, we inevitably reach a point where the "ought" has to be presupposed, and this is the presupposition of the basic norm.
In Kelsen's argument, P stands for the fact that legal norms are "ought" statements, and Q is the presupposition of the basic norm.
And we can see why: because legal validity is determined by the content of the basic norm that is actually followed in a given society.
Consider this question: What is the content of the basic norm that one needs to presuppose in order to render positive law intelligible as a normative legal order?
After all, the basic norm is a presupposition that is logically required to render the validity of law intelligible.
Suppose, for example, that in a given legal system the basic norm is that the constitution enacted by Rex One is binding.
This is basically what was meant earlier by the comment that Kelsen had no option but to admit that the validity of the basic norm is conditional on its efficacy.
But in both cases, as, in fact, with any other normative system, the presupposition of the basic norm is logically required only of those who regard the relevant norms as reasons for their actions.
