Byzantine views on knowledge are strongly influenced by late antique Neoplatonic Aristotelianism. A basic assumption in this tradition is that the nature of cognitive states is dependent on the nature of the cognitive objects (which have independent existence). Thus, the possibility of knowledge is secured by the existence of knowable things. Modifications of the Neoplatonic views are sometimes prompted by religious considerations, but these are more to do with emphasis than with content. It was strongly emphasized by the Byzantines, for instance, that God’s essence is beyond knowledge. Likewise, the Platonic theory of recollection was repeatedly condemned because it seemed to entail the soul’s pre-existence; on the other hand, the idea that the soul at birth is a tabula rasa was in conflict with the Christian doctrine that it is created perfect, and therefore Aristotle’s theory of concept formation was interpreted (e.g., by Eustratios of Nicaea) in a way that allowed for rational principles to be innate. In fact it is not uncommon to find in Byzantine writers rationalist accounts tracing the source of knowledge to innate soul-principles side by side (or nearly so) with endorsements of empiricist views suggesting that the first principles of knowledge are constructed from the individual forms of things.
Leo Magentenos (Leōn Magentēnos) was a Byzantine commentator on Porphyry’s Isagoge and Aristotle’s Organon.
Byzantine natural philosophy is heavily dependent on that of late antique Neoplatonic Aristotelianism, especially in the idiosyncratic form it took in the works of John Philoponus. In this tradition, nature is considered to be an inner principle of change (kinēsis) and stability (stasis), and natural philosophy is the branch of theoretical philosophy that studies such entities as are subject to change in accordance with nature, in contradistinction to mathematics and theology, the objects of which are exempt from change. The views of the late antique philosophers were mostly followed by the Byzantines as long as they were not perceived as contrary to the Christian faith. One view that was shared by most of the former but none of the latter is the view that the world is eternal. The Byzantines followed Philoponus in rejecting this view, rather than trying to harmonize it with creationism, as Proclus and others did. They also generally rejected views which seemed to entail it: thus the Aristotelian doctrine that the heavens are composed of an imperishable kind of body met with no support in Byzantium. Other features of Aristotelian-Ptolemaic cosmology were, however, readily accepted: the world according to most Byzantine writers is a system of nine nested spheres rotating at various speeds and in different directions (the ninth sphere being responsible for the diurnal motion from east to west) around the sublunary realm, where fire, air, and water form concentric layers with the small spherical earth at rest at the center. These elements are involved in a continuous cycle of transformation into one another, by virtue of each possessing one of the active qualities of hot and cold and one of the passive qualities of dry and moist. Some Byzantine writers, who found fault with Aristotle’s theory of place, also lent a willing ear to the Stoic cosmologist Cleomedes’ arguments in favor of the existence of extracosmic void. Philoponus’ influence is also obvious in the field of psychology, where most writers subscribe to an interpretation of Aristotle which leans strongly toward dualism: according to it, the lower soul faculties are inseparable from the body, but the rational soul, although dependent on the human body for some of its activities, is wholly separable from it in substance, and thus immortal.
Theodore Metochites (Theodōros Metochitēs, 1270–1332) was a Byzantine statesman, author, philosophical scholar, and patron of the arts. His philosophical works include paraphrases of Aristotle’s natural philosophy, an introduction to Ptolemaic astronomy, and a collection of “philosophical and historical” essays. A leitmotif running through the latter as well as his numerous speeches and poems is his insistence on the instability of things in the sensible world. This has ethical consequences: the proper demeanor under these conditions is to rise above the tide of joy and grief by means of unceasing reflection. It also has epistemological consequences: according to Metochites, nothing can be known with certainty outside the field of mathematics. Since we ourselves are part of the sensible world, our intellectual capacity is limited: this is why even the truths of Christianity cannot be the object of knowledge but only of faith. Without doubt, Metochites’ reflective practices, arguing pro and contra, and his vaguely skeptical theory of knowledge reinforce each other. He has often been regarded as a prime exponent of Byzantine humanism (although the very concept has been controversial). Indeed, it may even be tempting to bestow on him the epithet “Renaissance man,” not only on account of his encyclopedism (spanning poetry and prose on the most diverse topics) and his quest for synthesis (trying to combine philosophy and eloquence as well as to integrate the contemplative and active lives), but also in view of his criticism of Aristotle, his sympathy for skepticism, his fideism, and his interest in literary self-representation.