In 1879, Leo XIII penned his encyclical Aeterni Patris. In this encyclical, he warns the leaders of the Church about the dangers of modern philosophical thought and the impact of that thought on current affairs:
Whoso turns his attention to the bitter strifes of these days and seeks a reason for the troubles that vex public and private life must come to the conclusion that a fruitful cause of the evils which now afflict, as well as those which threaten, us lies in this: 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. (par. 2)
Clearly, the increasing secularization of society starting with the Reformation and Enlightenment, the changing form of government in many European and North American countries, and the rise of socialist and atheist influences in the nineteenth century provided enough cause for concern to the hierarchy of the Church. Citing Saint Paul’s letter to the Colossians, he reminds the pastors of the Church of the tendency of the faithful “to be deceived and the integrity of the faith [to be] corrupted among men by philosophy and vain deceit” (par. 1). In response to the threat of modernism, Leo XIII proposes a return to the teachings of St. Thomas Aquinas, the “Angelic Doctor.” What would be of principal importance to the efforts of Christian philosophers was the epistemology of Aristotle, which greatly informed Thomist philosophy.
Aristotle’s epistemology grew out of a dialogue of sorts between two opposing views of truth: spiritualism (particularly, Platonism) and materialism. Platonism held that reality exists in the world of thoughts or ideas rather than the physical world of things. The real world is the spiritual realm, and all things that find expression in the material world (phenomena) are simply reflections of perfect, innate ideas in this spiritual realm (noumena) (Turner sec. III, par. 1). For Plato, the soul has always existed and was once in the presence of “the Good” or “the One” (Bonsor 28). In this spiritual reality, the soul saw the whole truth. Through some twist of fate, souls have become trapped in the human body and have forgotten the realm of ideas or spiritual forms. Entombed as we are in this physical body, we see the material world, which is only a reflection of the spiritual reality that transcends it. To regain the truth, we must turn inward and, through introspection, recover our knowledge of the real.
The materialist view, in Democritus, predated Platonism (Gutberlet par. 1) but is reflected also in the thought of the Epicureans and the Stoics. Both schools arose following the death of Aristotle (Ryan par. 2). The materialists believe in the world known to us through the senses. To them, all knowledge of which we are capable comes to us through the senses. We process this sense data and come to understand our world. To the materialist, nothing else is necessary to know and understand ourselves and our world. Whereas truth to Plato exists in the realm of ideas, the materialists see truth more as a process of the brain. Materialism results in a mechanistic view of the universe, a view with which many empiricists today would feel comfortable.
Aristotle’s epistemology unifies the world of matter and the world of forms, thus reconciling the epistemological difference between the materialists and Platonists. He accomplishes this unification, in a sense, by taking from each view that which conforms to human experience. He agrees with the materialists that all knowledge begins with the senses. He does not accept Plato’s belief in innate ideas. Children are born with no concepts whatsoever in their minds. Through interaction with the world of the senses, children learn about the world.
Aristotle departs from the materialists in that he sees the intellect as being transcendent to the material world. Over time and through experience, children analyze sense data and form concepts. They come to understand that the thoughts and ideas they have, particularly imaginative thought and dreams, are somehow different than the material world. Aristotle also notes that the human capacity for language differs from the communication of animals. Whereas animals have a limited set of signals, human speech allows communication of complex ideas, ideas that are based on reality but can convey more precisely that which is essential versus that which is not. The intellectual capacity to differentiate between what is essential and what is irrelevant is what makes scientific thought possible.
By combining knowledge derived from sense data and the transcendent intellect, Aristotle’s thought bridges the dichotomy set up by the materialists and the Platonists and provides a unified epistemology in which both spiritual and material worlds exist and inform each other. St. Thomas Aquinas later took this “moderate realism” of Aristotle (De Wulf sec. II.a. par. 2) and applied it to Catholic theology, with some adjustments to render Aristotle’s thought compatible with the doctrine of the faith (for example, the coeternity of the First Mover and the material world, which conflicts with the Catholic doctrine of creation ex nihilo).
The rise of empiricism and the general aura of skepticism in the seventeenth century led again to a sharp division in human thought, a divergence away from the reconciliation proposed by Aristotle. The extremes of this division are exemplified in the works of David Hume and Immanuel Kant. Hume’s materialist approach was to “‘reject every system…however subtle or ingenious, which is not founded on fact and observation’” (Morris sec. 3 par. 3), what he referred to as a “mitigated scepticism” [sic] (sec. 8 par. 7). Immanuel Kant, on the other hand, opted for a subjective idealism, claiming that the scientific theories we derive are not drawn from sense data but from categories in our mind and are projected on to sense data.
In hindsight, the developments that followed this renewed skepticism of the Enlightenment period were easy to predict: rejection of religious authority, moral standards, and ultimately, of God’s very existence.
Leo XIII could not fail to recognize the dangers posed by skeptical modern philosophy, and he pointed to its inevitability. In his encyclical Aeterni Patris, the Holy Father notes, “For, since it is in the very nature of man to follow the guide of reason in his actions, if his intellect sins at all his will soon follows; and thus it happens that false opinions, whose seat is in the understanding, influence human actions and pervert them” (par. 2). Behavior follows reason, however deficient that reason may be. The danger inherent in the “turn to the subject” in modern philosophy was its tendency toward isolation and solipsism, toward a belief in the individual as the sole determiner of truth.
At the same time, Leo XIII undoubtedly recognized the validity of science and the need to reconcile faith and scientific fact. Faith and science could not contradict each other. So he sought to reconcile the two epistemologies rather than raise one above the other. The modern empirical trust in sense data and the subjective application of the mind to its analysis hearken back to the beliefs of the materialists and their belief in the ability of the human mind to analyze sense data and come to know the truth about the world. This similarity of epistemologies, perhaps, was not lost on Leo XIII, who recommended to the Catholic faithful a return to the epistemology of the original reconciler of the materialist–spiritualist dichotomy, Aristotle, through the works of St. Thomas Aquinas. The Holy Father notes that “the assistance of the Greek philosophy maketh not the truth more powerful; but, inasmuch as it weakens the contrary arguments of the sophists and repels the veiled attacks against the truth, it has been fitly called the hedge and fence of the vine” (par. 7). He recognized in the idealism of Kant the hint of Sophism. He recognized in the empiricists the skepticism of the materialists. Once again seeking to reconcile the materialist and spiritualist epistemologies, he turned again to the “hedge and fence of the vine,” the epistemology of Aristotle as represented in Thomistic philosophy.
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