Until recently, theories of vision were divided into two camps. On the one hand were those who believed in extramission (emissions from the eyes to the object), and on the other were those who argued that vision occurs through intromission (emissions from the object to the eyes).
In the 5th century BCE, the pre-Socratic philosopher Empedocles (490–430 BCE) argued that vision occurred when light issued from the eyes (extramission).
In the following century, the philosopher Plato (c. 420s–348/347 BCE) also believed generally in extramission, although he also supported the notion of intromission, such as when, in the Phaedrus (251c), he describes how when gazing on the beauty of a young boy, the beholder ‘receives the particles which flow thence from it’.
Aristotle also argued in favor of intromission (On Sense and Sensible Objects, 438a), but at times also supported the notion of extramission, such as when he refers to ‘the ray leaving the eye’ (On the Soul, 435a).
The issue was not resolved until the Middle Ages when the Moslem scientist Alhazen (Abū ‘Alī al-Hasan ibn al-Hasan ibn al-Haytham, c. 965–c.1040 CE), writing on optics, declared in favor of intromission, which is the prevailing view among scientists today.
However, despite Alhacen’s statement, the notion of extramission did not disappear. In the 15th century, Leonardo da Vinci remained convinced of its validity. Moreover, studies show that many people today, especially children and adults fearful of the ‘evil eye’, continue to believe that something leaves the eye when we see.
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