Of the three major approaches to Epistemology, (also known as “Major Logic,” or “Criteriology,” i.e., that philosophical discipline which studies the theory of knowledge), two constitute opposite, erroneous poles, while the third strikes a happy medium between them.
Materialism has for its epistemological foundation the natural sciences only. “For materialists, all truth must reduce to what can be observed by our senses or inferred from such observations to be material. Hence usually materialists recognize natural science as the ultimate science and the most reliably true form of human knowledge.” The Greek Stoics and Epicureans were materialist in their epistemology, as are the modern scientific positivists (or empiricists), who see in the scientific method the only sure criteria for knowledge.
This approach to knowledge is sensist in that it considers only the senses as reliable conduits of knowledge. In effect, this denies the power of the intellect to know truth and makes the process of abstraction something utterly unreliable. If the thing cannot be weighed, touched, smelled, seen, or observed in any other strictly sense manner, we simply cannot know it.
On the opposite pole from the materialist notion of epistemology, there is the spiritualist approach to knowledge. This criteriology “defends the existence of non-material or spiritual reality and either denies the reality of the sensible, material world or reduces it to an illusory or shadowy existence or at least puts no trust in the certainty of knowledge based on the senses.”
Included among proponents of this view are Parmenides, Plato, and the the Neo-Platonists, such as Plotinus, whose work was influential on St. Augustine (“Thou procuredst for me, by means of one puffed up with most unnatural pride, certain books of the Platonists, translated from Greek into Latin…. But having then read those books of the Platonists, and thence been taught to search for incorporeal truth, I saw Thy invisible things, understood by those things which are made”) Indeed, the patristic period in general, in those fathers who willingly embraced any philosophy at all, is marked by its Platonism. Thus, Father Ashley can say that the Neo-Platonists “strongly influenced Christian theology.”
Grounded on the Ultra-Realist metaphysics of Plato (ideas —forms — have a reality independent of their embodiments in things.), Neo-Platonism “holds that certain knowledge can be derived only from innate ideas recovered by introspection.” This view forms a pole opposite the materialist because of its general mistrust of sense knowledge. Whereas the materialist trusts only sense knowledge, the Platonist trusts only spiritual knowledge which has come to him by a process of introspection, not of abstraction based on things perceived by the senses.
Pagan notions that were embodied in Platonic thought (e.g., transmigration of souls, the eternity of the material universe, a disdain for bodily things which would deny the goodness of creation or the Incarnation) were cast aside by the orthodox fathers. “Yet the negative Platonic attitude to the body and the notion that truth can be arrived at only by introspection had a distorting tendency on Christian thought throughout Patristic (to about 600 AD) and Monastic (600 to 1200 AD) theology.” Examples of this “distorting tendency” in the patristic era can be found in Originism, Manicheanism, and Gnosticism in its various Christian manifestations. Even Arianism, with its attempted baptizing of Philo’s Platonic Demiurge, has Platonic roots. St. Anselm’s ontological argument for the existence of God, beginning as it does with an idea in the mind and postulating an a priori knowledge of God, would be an example of the deleterious effect of Platonism in monastic theology. St. Thomas would later dismiss the possibility of such a priori knowledge of God, and give five proofs using an aposterioristic methodology.
The happy medium between these two criteriological outlooks can be found in the moderate realist approach of Aristotle, which was later “baptized” by Boethius and, especially, St. Thomas Aquinas, O.P. The guiding principle of Aristotelian-Thomistic epistemology is nihil est in intellectu nisi prius fuerit in sensu. “He [Aristotle] rejected Plato’s innate ideas and agreed with the materialists that all valid human knowledge must be derived from and be tested by sense experience. [The human mind as it comes into this world is a tabula rasa awaiting sense impressions in order to gain knowledge.] On the other hand he also rejected the materialists’ denial of the existence of spiritual reality, because he argued that natural science based on sense experience demonstrates the existence of a First Cause that is spiritual and of a human intelligence that is also spiritual, although it depends on the First Cause for its existence and requires the body and its senses to arrive at truth. “
Having a regard for both sense knowledge and spiritual realities, Aristotle developed the science of metaphysics, which studies the relationship between the two. It is this appreciation of both spiritual and material realities in “the Philosopher” that led St. Thomas to find in his philosophy a worthy handmaid for Christian Theology. The mysteries of the Faith are more adequately explained in this hylomorphic system which respects both form (a “spiritual,” i.e., super-material reality) and matter. The Incarnation was an act of God, a profoundly spiritual event; yet, it was the embodiment of God, his taking of flesh. Sacraments are outward (material) signs instituted by Christ to give (spiritual) grace. The Mass is a sacrifice entailing the offering of a true Victim, who is present bodily, and who is bodily consumed by Priest and faithful alike, but by whose reception, mens impletur gratia “the mind is filled with grace.” A worthy philosophical approach must have a place for both material and spiritual realities, neither despising the one, nor discounting the other. Aristotelianism provided such an outlook for St. Thomas. It “made it possible for him to give a more adequate account of certain essential Christian convictions: (a) God as creator and the reality of creation (the real distinction of essence and existence in creatures, their identity in God); (b) a non-dualistic anthropology; (c) ‘Grace perfects nature.'”
Pope Leo XIII’s revival of Christian Philosophy, as advocated in his 1879 encyclical, Aeterni Patris, was concerned with undoing “the bitter strifes of these days” whose “fruitful causes” were these: “that false conclusions concerning divine and human things, which originated in the schools of philosophy, have now crept into all the orders of the State, and have been accepted by the common consent of the masses.” Before explaining why his revival chose Aristotelian epistemology (indeed, it chose philosophia perennis in its integrity as embodied in the doctrine of St. Thomas) to undo the errors of Kant and Hume, we would do well to review what those errors are.
“Immanuel Kant (1724-1804) proposed a form of idealism that held that although we cannot know the material world in itself, we can form general scientific natural laws about it as hypotheses that fit our sense experiences.” What mattered to Kant was the consistency that existed between categories that are innate in the human mind (like Plato’s innate ideas), and the reality that exists outside it, the reality we experience. “Truth is the conformity of mind to reality,” as asserted by the wisdom of the ages is replaced, in this system with, consistency between innate categories in the mind and the phenomena, i.e., the appearances we experience. Ever elusive, though, is a knowledge of the thing in itself (Ding an sich), or noumena. Kant denied our capacity to know things as they really are.
Though an Empiricist —which, one would think, should make him sensist and therefore positivist in his epistemology —”David Hume (1711-1776) argued for skepticism even as regards natural science, since, he claimed, the notions of cause and effect simply reflect our expectation that things will go on as usual, but they are not based on any sense data, since our senses only show us that one thing happens after another, not that one is the cause of the other.” Undermining the very notion of causality and the mind’s ability to discern it, Hume completely destroys the foundations of the scientific method, inasmuch as that method relies on observation and a process of logical induction.
Kant and Hume have gone beyond the epistemological errors of their respective idealist and materialist precursors from antiquity. The ancients accepted that the mind could know, erring on opposite extremes as to what constitutes the proper criteria of knowledge. Their modern counterparts, however, deny man’s ability to know reality. That said, Kant and Hume do advocate the worst elements of the opposing, perennial philosophical seductions of idealism and materialism. As these extremes were corrected by Aristotelian epistemological method in the day of St. Thomas, so today, a return to the doctrine of the Angelic Doctor will keep students from falling into one extreme or the other, sensist or spiritualist, no matter who the particular advocate of the error may be. Such, we surmise, was the intention of Pope Leo XIII in “restoring the renowned teaching of Thomas Aquinas and winning it back to its ancient beauty.”
 St. Augustine, Confessions, Book 7. (Pusey’s translation.)
 Ashley, op. cit.
 Ibid. (Italics in original.)
 St. Thomas Aquinas, Hymn, O Sacrum Convivium.
 Ashley, op. cit.
 Pope Leo XIII,Encyclical, Aeterni Patris, No.2.
 Pope Leo XIII,Encyclical, Aeterni Patris, No.27.Knowing how to handle such situations — with more suavity than the baker — can help in any number of apologetical situations.