In the Dioptrics, Descartes remarks that his theory of the instantaneous mechanical transmission of light will deliver “your spirit from all those tiny images flitting through the air, called intentional species, that so much exercise the imagination of philosophers (AT VI 85, CSM I 153–54). In this and other passages where he uses “intentional species,” the tone is invariably ironic, even disdainful, and mentioning them serves as foil to his own conception of how the things of the world communicate their presence to the senses.

Intentional species was a topos in Western medieval Scholasticism that produced a family of theories and interpretations operating within a complex of empirical and theoretical concerns. How, within the limits of Aristotelian metaphysics, physics, and psychology, does the sensation of material things and their properties occur, and how does sensation prepare the intellectual apprehension of them? The paradigm example was visual, the perception of color. For Aristotle color is the proper object, the proper sensible, of the visual sense; it is accessible in principle to any animals with eyes. How does this quality of real-world things come to be in sensation? Aristotle, in common with most though not all ancient theorists, thought that the effect proceeded from the thing to a more or less passively receptive eye. His physics of actuality and potentiality conceived four conditions needing to be fulfilled so that vision might occur: (1) a potentially visible material thing; (2) an eye (in a living, awake animal) with the potential for seeing; (3) a transparent medium between thing and eye; and (4) light. Light, rather than reflecting off the object and traveling to the eye, activates the medium's transparency so that it allows the active color quality in the physical thing to be communicated through the medium to the eye; in the eye, the communicated quality produces the same activity that exists in the physical object, though without that object's matter. This activation is seeing proper.

The spareness of Aristotle's explanation led Western Scholastic thinkers, beginning in the thirteenth century and extending into the seventeenth, to explicate the process further, with assistance from theories and concepts worked out by Islamic Aristotelianism.