One response to the Kantian (or broadly Thomistic) critique of the ontological argument is to say that it confuses a concept as such with a concept as equivalent to essence; or that they confuse the intelligible structure of things as known with their ontological intelligible structure. When we say that existence adds nothing to a concept, this is true of the concept as such or of the intelligible structure of things as present in an intellect, but it is false when said of the intelligible structure so far as it belongs to the thing itself – were this not the case, then nothing could actually exist with an intelligible structure.
More briefly, there seems to be a confusion between extrinsic or paradigm forms and intrinsic causes. The blueprint as blueprint is never a real building, but it is equivalent to an intrinsic form that does come to exist. Kant is right that a real building adds nothing to the blueprint, but this is clearly not the whole story.
The Parmenidean claim that the to be and the to be thought are the same is, at least for us, only (at best) a statement about essence, which has a real ambivalence to conceptual and real existence. But even this statement admits of significant qualification: the ambivalence co-exists with the a division of form into paradigm and intrinsic cause.
Seen from this angle, the Anselmian proof is the claim that the equivalence of thought and essence requires that there be some maximal intrinsic form corresponding to a maximal paradigm. But this seems like a difficult case to make, given that there is no necessity for a paradigm to have a real intrinsic cause. Blueprints are not necessary indications of buildings. Nevertheless, of we had a general theory that existence was nothing but a participation in an essence or paradigm case (as Plato seemed to have thought) then the knowledge of a maximal paradigm would immediately be known to exist, if by maximal we meant “non-participated”, which, again, seems to be what Plato would have thought.
And so an ontological argument that would make more explicit some Platonic presuppositions might go like this:
1.) I know the maximal paradigm. (true by reflection, or coherent use of the term)
2.) The essence that corresponds to the maximal paradigm, if there is such an essence, is non-participated. (From Platonic and Thomist principles. The premise is given even in STA’s critique of Anselm’s argument in ST 1. 1.2.2.)
3.) Among the essences that correspond to paradigms, to exist apart from or after the paradigm is proper to the participated. (Definition of “participated”, which also follows from an account of how generation happens.)
4.) Therefore, I know that the essence which the maximal paradigm corresponds to exists.