The theory of vision changed in the 16th-century when intramission was rediscovered in Europe.
The perception of the world, the perception of reality itself, is a reflective process that is mediated by objects in the world. There is a threefold structure to everything we see — there is a source of light, there are objects that reflect the light, and there is our eye that captures the reflections.
But this description is a radical one, at least from the perspective of Elizabethan England. It was as late as 1572 when the theory of intramission — that rays of light enter the eye to produce vision — was introduced in Europe. Five hundred years earlier, in Cairo, Alhazen (Latinized from Hasan Ibn al-Haytham) wrote the most significant new scientific theory of optics and visual perception since Ptolemy and Galen. He worked out the physics of reflected light, the functions of the parts of the eye, and the role of the brain in producing images.
Before Alhazen’s theory of intramission, the classical understanding of vision prevailed. Going back to the fifth century BCE, Empedocles imagined that a fire burned within the eye, lit by the goddess Aphrodite. Rays of light shone outward in straight lines like the beam of a flashlight. The theory of extramission was echoed and developed by Aristotle, Ptolemy, and Galen in the following centuries.
The way this works is peculiar. The eyes, lanterns in the shadow, burn to illuminate the world. The rays of the eye give definition to the world — there is a binary relationship between the mind and the world. The eye, a sun, produces its own light and makes it possible for the mind to perceive objects. In this way, a person turning their gaze to some thing defines that thing by illuminating it. Perception is active and unmediated by the object.
Intramission and extramission are not only two very different ways of understanding the optics of sight, but also very different models of subjective experience. The ancient idea of extramission emphasized an active rather than a passive model of perception. Rays of light emerge from the eyes. The eyes in turn are compelled by reason to illuminate the objective world. This complemented Neoplatonic ideas about the soul actively reaching out through vision to grasp divine truth. Apprehending the objective world is an active process through and through.
The beauty of color is also the outcome of a unification: it derives from shape, from the conquest of the darkness inherent in Matter by the pouring-in of light.1
This model of vision led to a metaphysics of metaphor. Vision creates meaning, defines reality. Ptolemaic optical theory included both physical and psychological components and operates on multiple levels, from material to spiritual.
To any vision must be brought an eye adapted to what is to be seen, and having some likeness to it. Never did eye see the sun unless it had first become sunlike, and never can the soul have vision of the First Beauty unless itself be beautiful.2
Renaissance thinkers often compared the eye’s structure to cosmic order. The spherical eye was seen as mirroring celestial spheres, the eye a micro-cosmos. The Neoplatonists, and the Renaissance humanists who followed them, likened the physical process of vision to spiritual contemplation. The eyes seek out a world, and what they perceive follows the contours of the seer’s own spiritual development.
The optics of intramission are much simpler. Light enters the eye and is perceived passively. The eyes are wells that capture reflected light from objects in the world. There is no spiritual affinity that enables perception. The eyes don’t define reality by illuminating it, they simply receive rays of light. From this, our brain works out the image and gives it over to thought.
BRUTUS No, Cassius, for the eye sees not itself
But by reflection, by some other things. (Julius Caesar, I.2.58-59)
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