Pope Benedict XVI made a priority in his theological career of emphasizing the absolute importance of retaining a robust sense of the doctrine of creation. Of the many ways in which that doctrine impacts or even determines other branches of Christian thought, perhaps none are so important as the connection between divine creation and human understanding.
For Thomas Aquinas, the fact that every reality other than God himself is creatura—created by God—permeates and informs everything from metaphysics to ethics to ontology to epistemology: He writes that “[God] produces the whole substance of things, presupposing nothing but rather being himself the source of all existence with the whole of himself.” 
One (of many) particular ways in which Aquinas takes up creation into a larger treatment of human affairs concerns his treatment of creation, knowledge, and truth, which treatment Josef Pieper describes brilliantly in his essay “The Negative Element in the Philosophy of St. Thomas Aquinas.”  Here, Pieper summarizes the negative elements of Aquinas’s ontology, explains those elements in light of divine creation, and clarifies how these elements impact human epistemology.
Aquinas taught that man’s essence consists in the compenetration of form and matter, a hylomorphic unity comprised of body and soul, the substantial form of which is the rational and immaterial soul created directly by God.  Man, the unity of form and matter, is the subject of knowledge—not some intellectual faculty within man, nor man’s rational soul; but man himself.
Furthermore, Aquinas taught that knowledge consists in a unification or symmetry between one’s intellectual faculties and reality, or being, or truth. Thus, Aquinas raises the question of how it is that man’s perception penetrates to the nature of things and is capable at all of real knowledge of the way the world is.
St. Augustine actually encapsulates well the argument that Pieper lays out in tracking the negative philosophy of Aquinas when he (Augustine) writes near the end of the Confessions, “We, therefore, see these things you have made, because they exist, but for you it is different: they exist because you see them.”  Pieper summarizes Aquinas’s equivalent perspective thus: “The essence of things is that they are creatively thought.”  Here, Aquinas touches on the meaning of St. John’s writing, “the Word was God.” As Pope Benedict XVI explains in his "Regensburg address," the profound meaning of this sentence is precisely this: God himself is logos, dynamic reason intelligible and communicable as reason: “The Christian picture of the world is this ... that at the most profound level it comes from the Logos. Thus it carries rationality within itself ...” 
This “reason” is the “word” (another translation of logos) through whom God creates “all things.” Ratzinger points out—and succeeds Thomas in doing so—that the structure and the “logic” and the rhythm of natural created order, then, evince and point back to this divine logos itself. As such, creation is intelligible to man, the creature who by virtue of his intellect and will is fashioned in God’s “image and likeness,” and whose mind can participate in and perceive the ordo rationis of the universe.
The very intelligibility of the essence of things derives from those things' creative origin in the Logos. It follows that things can only be known because they are creatively thought and willed into being by the divine mind; insofar as things correspond to the archetypal Word of God, insofar as the copies resemble or image the divine blueprint, those things are “true” and thus can be known by man.  “Things have their intelligibility, their inner clarity and lucidity, and the power to reveal themselves, because God has creatively thought them,” Pieper writes.  Man’s knowledge is “true” knowledge—it corresponds to “the way things really are”—insofar as it receives its measure from created things and corresponds to those things’ created (and therefore objective) natures. Aquinas summarizes this dependent relationship aptly: “Knowledge is a certain effect of truth ... indeed of the truth of things.”  Reality is intelligible and knowable to man only because it is creation and emanates from the mind of God as from an architect.
At the same time, a proper understanding of divine creation also tempers and situates man’s intellectual pursuit, contextualizing it and preserving it from erroneous presumptions concerning its scope and power. For the same rootedness in the Logos that makes things intelligible to man also makes the essence of those things incomprehensible to him. If man’s knowledge is measured by its correspondence to the objective nature of the created thing, the thing itself is both measured by man and receives its measure from God’s mind. So the created thing occupies the middle of a three-point spectrum, at either end of which is God and man; the former imparts being (existence) through his Logos, in light of which ontological clarity (possessing its own logos) the latter, whose mind can indeed “measure” the objective nature of the created thing, can know. But man, a creature himself of finite faculties, cannot “measure” the relationship between the type and its archetype, formally speaking; he cannot comprehend the essence of the thing, because the essence is an expression of, and has its ultimate source in, the infinite and boundless light of the divine mind. In this sense, Pieper writes, “Not only God Himself but also things have an ‘eternal name’ that man is unable to utter”! 
Creation is “too rich” to be comprehended or known perfectly by man. Creation’s “knowability,” its inexhaustibility, and its incomprehensibility are all grounded in its singular source: the Logos. Thus the seeming paradox of Aquinas’s negative philosophy: The same great thinker who wrote millions of words of intellectual discourse also writes that man cannot grasp the essence of a single fly, which essence remains in itself unfathomable to him. The essence of all creation is rooted in the absolutely creative mind of God, and the divine mind cannot at all be plumbed by man.
Nor is the import of this teaching of Aquinas’s merely academic, serving as a helpful methodological principle in the service of sound epistemology. For man, too, is a creatura, and everything that applies to the aforementioned “things” is true of man as well. (“Each person is God’s own idea,” Ratzinger writes.  Elsewhere he states that “each of us, each individual human being, realizes the one project of God and has his or her origin in the same creative idea of God.” )
What this means is that human relationship itself admits of the same paradoxical relation as any other creatura: Creation explains why the human person is intelligible to himself, and to some degree can make sense of other persons. But the depth of interpersonal relationship cannot possibly be plumbed, and one cannot possibly “know” a person comprehensively or completely (perfectly). Even in the deepest and most mature of loves, there remains a region of mystery in the other, inaccessible to every effort to penetrate: The quest for intimacy through union with the other is inexhaustible precisely because man is a creature.
C. S. Lewis spoke movingly of his feeling of sehnsucht, his recognition that his own desires for intimacy and communion transcend the created world’s ability to satisfy. Here one begins to touch on the reality of erotic love and its transcendent horizon; Pieper writes in his essay “On Love” that “what happens in erotic love is thus not gratification but an opening of the sphere of existence to an infinite quenching that cannot be had at all ‘here.’” 
It is this “region of mystery,” inaccessible to man yet yearning for fulfillment, which bespeaks the deepest truth about man: He is fashioned (created) by God for communion in Himself. Only in this relationship does man discover his own true dignity and the root of his flourishing. “Human beings are most profoundly themselves when they discover their relations to their Creator,” Ratzinger writes.  Augustine writes, “in your Gift [the Holy Spirit] we find rest, and there we enjoy you. Our true place is where we find rest.”  And the 20th-century Camaldolese monk Aelred Squire sums this dynamic up beautifully, articulating a truth that contemporary culture so badly needs to hear:
A good deal of frustration in human relationships results from a failure to recognize that there is an inescapable element of solitude in every human life, which not even marriage or the most intimate of friendships can evade. That is, indeed, something which each person must respect in themselves and in others as the most precious thing of all about them. It is something that cannot be given away, for in its ultimate depths there must be an aspect of every human soul which is virginal towards God.
Most of the more terrible kinds of human unhappiness arise from a refusal to recognize this fact or the desire to evade it. It is the real root of each person’s individual dignity, however, and the true source from which his greatest joy will flow, when the love of this unique love becomes fruitful at the level of his being which is accessible only to God. 
 From Thomas Aquinas, Quaestiones Disputatae de Potentia 3. 1 in Aquinas: Selected Philosophical Writings, translated by Timothy McDermott (Oxford: Oxford University Press 1993), 255
 Included in Josef Pieper, The Silence of St. Thomas (South Bend: St. Augustine’s Press 1999), 45-71
 See Aquinas’s Quaestiones Disputate de Anima 7
 St. Augustine, Confessions, translated by Maria Boulding, OSB (New York: Vintage Spiritual Classics 1997), XIII, 38
 Pieper, 51
 Joseph Cardinal Ratzinger, God and the World: A Conversation with Peter Seewald (San Francisco: Ignatius Press 2002), 139
 I will qualify this statement shortly by drawing in Aquinas’s thought on the incomprehensibility of created things.
 Ibid., 55-56
 Thomas Aquinas, Quaestiones Disputatae de Veritate, I, I
 Pieper, 65
 God and the World, 75
 Ratzinger, ‘In the Beginning...” A Catholic Understanding of Creation and the Fall (Grand Rapids: William B. Eerdmans Publishing Company 1986), 45
 Josef Pieper, Faith, Hope, Love, (San Francisco: Ignatius Press 1986), p 252
 In the Beginning, 48
 Confessions, XIII, 10
 Aelred Squire, Asking the Fathers (London: SPCK, 1994), 115