If Mary’s rational soul and so her capacity for sin was only infused into her body post conception on what grounds could such an immaculate conception take place? Duns Scotus used Franciscan voluntarism to ‘solve’ his problem where the will rather than the intellect takes precedence. Which taken to its logical conclusion causes all kinds of problems for the principle of non-contradiction [God could create a square circle if He wanted to, perfection could be less than perfect if He wanted to be. Etc.] If St. Thomas (and Dun Scotus) et al had modern biology to base their philosophy and theology upon then there would have been no debate at all. But then if we diden’t have franciscian voluntarism its unlikely we would have had nominalism, conceptualism and the general nuttiness of modern philosophy and theology.
So the complaint is that if the will has "precedence" or "precedes" the intellect, lots of bad thing follow. Philosophically you get God able to create a square-circle, or be less perfect, or whatever. Historically, Scotist voluntarism "causes" (read Brad Gregory for an explanation of this causality) nominalism, conceptualism, and everything the one making the claim doesn't like about the modern world.
The philosophical claim underlying all this seems to be that according to Scotus, possibility and impossibility are dependent on the divine will.
The passage in Scotus to examine is I d. 43 of his commentaries on the Sentences, the Lectura, Ordinatio, or Reportatio.
One thing we find is that the will plays no role in whether something is possible or impossible. For Scotus, possibility and impossibility is a feature of terms (or natures, essences) which are generated by the divine intellect. The divine intellect generates say, 'rational' and 'animal'; there is no repugnance between these terms, so the species human being is possible. The terms 'square' and 'circle' are repugnant, so a square-circle is impossible. This is even true if per impossible, God did not exist. If God did not exist, and neither did anything else, a square-circle would be impossible because the terms are repugnant, and human being would still be possible because the terms are compatible. This is what Scotus calls logical potency, and the result of it seems to be that modality is grounded in things themselves or their essences, rather than on God or some feature of God. Now on this tricky point Scotus actually says that possibility is "principiative" from the divine intellect. The idea is that while the terms are repugnant or non-repugnant based on their natures, for there to be any terms or essences at all there must be the divine intellect to generate them.
So whatever other philosophical problems voluntarism might have, at least for Scotus we are not in danger of a world of square-circles or impossible objects walking the streets, or God making himself not-God. Possibility and impossibility arise from the relation of the divine intellect and its thinking about essences.
