The most influential theories of illumination explain certain features of our knowledge by developing an analogy with ordinary sensory vision and the role played in it by light. According to theories of this sort, our knowledge of necessary and immutable objects and truths requires the activity of a kind of intelligible light illumining objects that are purely intelligible, thereby making them 'visible' to our mind. Plato held that this light comes from the Form of the Good, Augustine that it comes from God, and others that it is intrinsic to reason itself.
The peculiar nature and behaviour of light has provided a model not just for theories of knowledge but also for philosophical accounts of fundamental features of reality. Neoplatonist cosmologies, for example, liken the genesis of the universe to the emanation of rays of light from a light source. Theories of illumination therefore can be metaphysical as well as epistemological. Both kinds of theory have their historical roots in Platonism broadly construed.
Platonist epistemologies characteristically focus on the fact that we understand certain immutable, eternal natures (for example, the objects of geometry, such as the circle and the triangle) and know certain necessary truths (for example, that the interior angles of a triangle are equal to two right angles). They argue that we cannot have acquired knowledge and understanding of this sort through sense perception since no objects of sense perception are of the right sort to account for the necessity and immutability distinctive of this sort of knowledge. They conclude that if such knowledge is to be possible, there must be necessary and immutable objects epistemically accessible to us by means other than sense perception. Epistemologies of this sort hold that these objects are purely intelligible and that we make epistemic contact with them through reason, intellect or mind. Our understanding of the nature of a triangle (for example) and our certain knowledge that its interior angles are equal to two right angles are to be explained by our being (or having been) directly acquainted with the relevant purely intelligible object(s).
The model for this epistemically fundamental notion of direct intellective acquaintance is sensory vision. Our access to purely intelligible objects by means of reason or mind is analogous to our access to ordinary visible objects through sight. According to this analogy, there are three essential components in both the sensory and intellective cognitive processes. First, we must possess the relevant cognitive power (sight in the case of sensory vision; intellect in the case of intellective cognition). Second, there must be an appropriate object for that power (a coloured material body; a purely intelligible object). And third, there must be an agent whose activity enables the cognitive power and its object to make contact. In sensory vision, this third component - the enabling agent of cognition - is light: corporeal light (from the sun, for example) illumines material objects, making them not merely potentially but actually visible to us. In intellective cognition, there must be an analogous intellectual light whose activity makes intelligible objects actually and not merely potentially intelligible to us. Our direct acquaintance with the nature of a triangle (for example) depends essentially, therefore, not only on our intellective powers and the existence of the relevant intelligible object(s) but also on the activity of an intellectual light.
The analogy between intellectual understanding and sensory vision is superficially compelling (our ordinary ways of talking about knowledge and understanding rely heavily on it), but the metaphor of intellectual light gives rise to difficult philosophical questions. What exactly is the nature of this intellectual light? Is it something ontologically independent of us or is it identical with or merely a function of our intellectual powers and capacities? Plato's famous analogy of the sun suggests that the intellectual light has as its source the Form of the Good:
Say, then, that it is the sun which I call the offspring of the Good, which the Good begot as analogous to itself. What the Good itself is in the world of thought in relation to the intelligence and things known, the sun is in the visible world in relation to sight and things seen.... [W]henever one's eyes are turned upon objects brightened by sunshine, they see clearly.... So too understand the eye of the soul: whenever it is fixed upon that upon which truth and reality shine, it understands and knows, and seems to have intelligence.... Say that what gives truth to the objects of knowledge, and to the knowing mind the power to know, is the Form of the Good.
Later Platonists develop and extend what we might call the supernaturalist elements of Plato's analogy. Augustine (§4), for example, typically describes the source of intellectual illumination not as the Form of the Good but as Truth itself, which he identifies with God. He holds, then, that our knowledge depends essentially on God's activity in our souls. For this reason, his view is typically characterized as a doctrine of divine illumination.
By developing Plato's analogy in an explicitly theological direction, Augustine intended to deepen it and weave its epistemological elements into his broad, theistic account of reality. But medieval philosophers attracted by Augustine's Christian Platonism saw two general difficulties in his doctrine of divine illumination. First, Augustine had suggested that in so far as any particular truth is necessary, immutable and eternal, it must be, or in some way be a part of, God, since God alone is truly necessary, immutable and eternal. But in that case it seems that in grasping a truth of this sort, ordinary human knowers are in direct epistemic contact with the divine nature, a state that Christian doctrine takes to be virtually unattainable by human beings in this life. Second, in so far as Augustine's account identified illumination with God's activity within our minds, it seemed to make genuine knowledge a product of supernatural intervention in our cognitive lives, and so not genuinely human at all.
Medieval philosophers and theologians typically try to steer a middle course between an interpretation of Augustine's doctrine of divine illumination that is unacceptably supernaturalist and an interpretation that leaves no room at all for divine activity in human knowing. Many preserve the doctrine essentially intact, introducing minor distinctions designed to blunt the most serious objections. Bonaventure (§7), for example, argues that our grasp of the eternal natures that are in God can be a matter of degree:
Since, then, certain knowledge belongs to the rational spirit, insofar as it is the image of God, it follows that in this knowledge the spirit attains to the eternal reasons [that are in God]. But since, as long as it is in the wayfaring state, [the rational spirit] is not fully deiform, it does not attain to them clearly and fully and distinctly.... It is to be granted, then,... that in all certain knowledge those principles of knowledge are attained by the knower. They are reached in one way, however, by the wayfarer, and in another way by him who enjoys the vision of God [in the next life].
But some important late medieval thinkers give the doctrine of divine illumination an essentially naturalistic interpretation; this is especially the case with those philosophers - such as Bonaventure's contemporary, Thomas Aquinas (§11) - who are committed to an Aristotelian analysis of mind and cognition. Aquinas argues that the intellectual light that makes immutable natures actually intelligible to us must be identified with a power in us, otherwise the resultant knowing would not be ours. That power, Aquinas claims, is the Aristotelian agent intellect, the power of the human soul that abstracts universal natures from the material and individuating conditions that characterize the presentations of sense perception. He adapts the terminology of the doctrine of illumination to fit the structure of his Aristotelian account, calling the agent intellect a 'light' and thinking of its activity of abstracting universal forms as a kind of 'illumination'. In doing so, Aquinas preserves the letter of Augustine's position if not its entire spirit.
This naturalistic rendering of the doctrine of illumination survives into early modern philosophy in the work of philosophers such as René Descartes (§7). The foundational role he assigns to propositions that are clearly and distinctly perceived to be true (or that are shown to be true by the natural light of reason) marks Descartes' theory of knowledge as part of the legacy of Platonist epistemological illuminationism.
Light's seeming propagation of itself in all directions from its source provides the model for emanationist cosmologies and related metaphysical accounts. Plotinus (§§3, 5), for example, described the emanation of all reality from the single, unified source of being (the One) as like the emanation of rays of light from a light source. Medieval Christian philosophers, who rejected emanationism as an account of the genesis of the universe, nevertheless incorporated the notion of emanation into their explanations of various aspects of the relation between creatures and their creator. According to Albert the Great (§4), for example, metaphysical universals are like rays radiating from God, the primary intelligence and creator of all forms. These universal forms, which are in themselves simple, fall on matter (producing particular individuals) and on souls (for which they are the principles of cognition). Henry of Ghent (§§2-3) argued that a creature's essence is related to its existence in the way a ray of sunlight is related to the light it possesses: the ray receives its light from the sun (and so is in that respect distinct from and dependent on it), but it receives it in such a way that the ray's very nature and essence is light (and so is in this respect not distinct from it). Accounts of this sort can be thought of as developing the metaphysical side of the metaphor of illumination.
See also: Chinul; Epistemology, history of §§1-4; Grosseteste, R. §3; Illuminationist philosophy; Medieval philosophySCOTT MacDONALD