The idea of a divine "illumination" in the mind occurs in both philosophical and religious contexts. Often it forms one of the links between the two types of thought, and sometimes it bears distinctly religious overtones even in its more philosophical applications. This is one of the characteristic features of the theory of illumination in the thought of Plato, where it played, for the first time in its long history, a major part. Plato, like many other thinkers, creative artists, prophets, and mystics, spoke readily of the sudden flash of understanding or insight in the mind as a flood of light (see, for example, his Seventh Letter, 341C, 344B). The image is, indeed, one that occurs naturally in many languages and is especially apt for the description of insight thought to have been achieved as a result of external aid of some kind, of an "inspiration." The language of inspiration is based on the entry of breath, and that of illumination on the entry of light into the mind. The Stoic tradition can be said to have developed the former analogy in its metaphysics; Plato was undoubtedly the father of the philosophical tradition to which the analogy of light is fundamental.
In his Republic, Plato employed the analogy of light and vision to describe the process of understanding or of knowledge in general (Books V–VIII). The mind's knowledge of the world of intelligible reality, of the forms or ideas, was held to be analogous to the awareness of material objects accessible to the eye's vision when illuminated by the light of the sun. Plato developed a detailed correspondence between physical and intellectual sight (Republic 507f.), according to which the mind corresponds to the eye and the form to the physical object seen; an "intellectual light" emanating from the supreme form, the Good, and pervasive of the whole intelligible world as well as the mind, corresponds to the sun. Understanding, in terms of this analogy, depends on the intellectual illumination of the mind and its objects, just as vision depends on a physical illumination of the eye and its objects.
A theory of this type, in one or another of many variant forms, became an essential part of a vast body of thought cast in Platonic molds. During the Hellenistic and Roman periods it was widely diffused and incorporated into Jewish and Christian thought. In the Hellenized Judaic milieu of Alexandria the divine wisdom was sometimes spoken of in terms of light, for instance, by the author of the book of Wisdom, who referred to it as "an effulgence of eternal light," which he interpreted as an image of God's goodness (7, 26). Thoughts of this kind found a place in the work of Philo and in the prologue to the Fourth Gospel. Middle-Platonist thinkers, such as Albinus, took the step—perhaps already hinted at by Plato in some passages—of placing the forms within a divine mind and, in effect, identifying the "intelligible world" with the mind of God. In this way a long and rich future was prepared for the theory of illumination within the body of Christian thought.
In Christian thought it is in the work of St. Augustine of Hippo that the theory of illumination is found in its most highly developed form. Like Plato, Augustine thought of understanding as analogous to seeing. Understanding, or intellectual sight, was therefore, he held, conditional on illumination, just as physical sight was; only here the light was the intelligible light that emanated from the divine mind and in illuminating the human mind endowed it with understanding. Understanding, in the last resort, was an inward participation of the human mind in the divine. The scope of illumination was further extended, at the cost of precision, in the work of the pseudo-Dionysius. His favorite designation for God, the absolutely transcendent One, was in terms of light. God is the intelligible light beyond all light and the inexhaustibly rich source of brightness that extends to all intelligence. His illuminating activity gathers and reunites all that it touches; it perfects creatures endowed with reason and understanding by uniting them with the one all-pervading light (De Divinus Nominibus, IV, 6). In true Neoplatonic fashion, the pseudo-Dionysius conceived of the cosmos as a hierarchically ordered system, descending in order of reality and value from its source, the One. Illumination, in general terms, is the means by which intellectual creatures ascend and return to unity, and the "hierarchy" (understood as extending through both the cosmos and the church) is defined as the divine arrangement whereby all things, participating in their measure in the divine light, are brought back to as close a union with the source of this light as is possible for them (De Coelestia Hierarchia, III, 1). In a more special sense, illumination is the second of three phases—namely purification, illumination, and perfection—of man's return to the One. In this more specialized sense the church's sacramental system and the grades in the ecclesiastical hierarchy concerned with its administration are agencies of divine illumination. Illumination is the intermediate stage of approach to God, between initial purification and final perfection (De Ecclesiastica Hierarchia, V, 1, 3). In the most restricted sacramental contexts "illumination" thus becomes synonymous, in accordance with an old Christian usage, with "baptism." In the work of the pseudo-Dionysius the theory of illumination was merged with an inclusive conception of the spiritual life formulated in the language of light and illumination.
The reputation enjoyed by Augustine and by the writings of the pseudo-Dionysius in the Middle Ages assured their views a long future. In the thirteenth century the rise of Christian Aristotelianism provided the first serious alternative theory of knowledge. In this there was no place for the intervention of a divine illumination as an essential constituent of knowledge. Knowledge was accounted for entirely in terms of mental activity and its objects, and no reference to God was necessary to explain it. Nevertheless, the lumen intellectuale of the mind was held to be a participation in the lumen divinum of the divine mind, since God was present everywhere, in the mind no less than in other things. In this way Christian Aristotelians, such as St. Thomas Aquinas, were able to endorse some characteristically Augustinian statements in spite of the fact that their theories of knowledge were built on a radically different structure. The Augustinian version of the theory of illumination continued to have a vogue among some thinkers of the thirteenth century, such as St. Bonaventure, and even later. It found echoes in the thought of some modern philosophers, such as Nicolas Malebranche. Increasingly, however, in the later Middle Ages and after, the language of illumination, especially as elaborated by the pseudo-Dionysius, became the special property of mystical writers and writers on the spiritual life.
Allers, R. "St. Augustine's Doctrine on Illumination." Franciscan Studies 12 (1952): 27–46.
Geach, P. Mental Acts. London: Routledge and Paul, 1957. Section 11 and the appendix include corrections of the standard account of Thomas Aquinas's theory of concept formation.
Gersh, Stephen. From Iamblichus to Erigena: An Investigation of the Prehistory and Evolution of the Pseudo-Dionysian Tradition. Leiden: Brill, 1978.
Jolivet, R. Dieu soleil des esprits. Paris: Desclée de Brouwer, 1934. A study of the theory of illumination.
Markus, R. A. "St. Augustine on Signs." Phronesis 2 (1957): 60–83. Includes a discussion of illumination in Augustine's theory of knowledge.
Marrone, Steven. The Light of Thy Countenance: Science and Knowledge of God in the Thirteenth Century. Leiden: Brill, 2001.
Pasnau, Robert. "Henry of Ghent and the Twilight of Divine Illumination." Review of Metaphysics 49 (1995): 49–75.
Pasnau, Robert. Thomas Aquinas on Human Nature: A Philosophical Study of Summa Theologiae 1a 75–89. Cambridge, U.K.: Cambridge University Press, 2002.
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