Scholars were divided between the extramission and intromission theories of vision until Ibn al-Haytham’s eleventh-century experiments largely settled the debate in support of intromission.

Through two opposing theories of vision—extramission and intromission—Euclid, Aristotle, and Ibn al-Haytham held that a form of radiation is emitted either from objects or from the eyes.

While Ibn al-Haytham largely settled the debate in the eleventh century, Aristotle supported the theory of intromission centuries before.

Before the eleventh century, the ancient Greek philosopher Aristotle supported the intromission theory, which held that objects emit a form of radiation that reaches the eyes.