fear that there will always be a 'fourth'—the divine essence in all Christian trinitarian understanding of God—insofar as one grasps, as Ruusbroec clearly does, that God's essence is dialectically self-manifesting and thereby is necessarily Father-Son-Spirit.
I believe, furthermore, that Macquarrie is correct to state that this Christian insight implies that 'natural theology' should also feel obliged to show the intrinsically incarnational and relational trinitarian structure of all reality. Unlike not only the Barthians but also the Thomists (with their distinction between 'natural' mysteries and 'supernatural' mysteries), the neo-Platonists (including, here, some Hegelians and some Whiteheadians, Rahnerians, and Tillichians) develop philosophical positions that attempt to show, minimally, the reasonableness of an incarnational, relational, and trinitarian understanding of all reality. Such an enterprise, to be sure, demands a full incarnational metaphysics of relationality.
In the meantime, precisely the Christian trinitarian (self-manifesting, radically relational) understanding of the Divine Reality so well expressed by Ruusbroec may suggest that even the most radically speculative form for trinitarian theology of the immanent Trinity can cohere with the realistic forms (narrative, doctrine, liturgy) for the economic Trinity. Even more important is the contemporary rediscovery of the importance of radical apophaticism for all naming of God—including an adequate trinitarian economic and immanent naming—as well as for trinitarian reflection in the analogical (and thereby also partly apophatic) theologies of Augustine, Aquinas, and Lonergan, and even for the more radical apophaticism of Ruusbroec and possibly Eckhart. Some further attention by trinitarian theologians to form-content relationships here could significantly aid what is for me a central question for all trinitarian theology: the relationship of trinitarian language to apophaticism. The key to that relationship is the relationship of form and content.
On hermeneutics: see Hans-Georg Gadamer, Truth and Method, rev. trans. J. Weinsheimer and D. G. Marshall, 2nd rev. edn. (New York: Continuum, 1994); Martin Heidegger, Zur Sache des Denkens