 But it would be unsatisfying to take inertia as primitive—to say that bodies in fact do tend to persist in their states, though there is no reason to be discerned in their nature why they do so. To say this would be to take it that Spinoza accepted the Cartesian principle as Descartes understood it, while rejecting the grounds Descartes offered for it, and without providing any substitute for it. This is dissonant indeed with the general tenor of Spinoza's  rationalism.[4]  Moreover, on this view, Spinoza would have taken the trouble to make explicit the PLMM Descartes' rules require, only to justify it by a direct and unelaborated appeal to a groundless principle of inertia, which seems, on Descartes' understanding of it, quite inadequate to do the job. Yet Spinoza evidently did think that the PLMM followed from the principle of inertia. This suggests that he had a different conception than did Descartes of both the nature and ground of that principle.