That Time Humanity Had No Idea What Light Was

This Summer, NASA announced its plans to launch a mission in 2018 to the sun—that big ball of light in the sky. The Parker Solar Probe will “swoop to within 4 million miles of the sun’s surface,” they say, “facing heat and radiation like no spacecraft before it.”

With projects like this, literally taking us to an important source for light on earth, it is hard to imagine that there actually was a time when human beings didn’t know anything about light. For most of our history, it appears we didn’t even understand the source of it.

As part of a book project of mine, I ventured briefly into the history of light. Initially, human beings thought light resided in the person, flowing out from an individual’s eyes and to an object. The light derived from us enabled us to see and understand the world. This theory was called the extramission theory of light; various philosophers had their own nuances on the idea. 

For Plato, for example, human eyes are our source of light, possessing fire (the pupil), and when that light is emitted from the eye it coalesces with daylight, allowing the light of the eye to see objects of the material world. This internal light also provided greater spiritual apprehension of the world. 

Christian Platonists, like Origen of Alexandria and Augustine, followed Plato’s extramission theory. For Origen, emission of light from the physical eyes was really a spiritual exercise, that is, the internal light was primarily about spiritual sight. Extramission theory of physical sight provided the groundwork for the extramission of spiritual sight and a divine sense.

Of course, Origen and Augustine also had a biblical reason for their views. A fairly explicit reference to the extramission theory appears in Matthew 6:22-23, in which Jesus says that “the eye is the lamp of the body. If your eyes are healthy, your whole body will be full of light. But if your eyes are unhealthy, your whole body will be full of darkness.” 

Despite extramissions dynasty, it wasn’t the only option on the street.

Aristotle (Plato’s student) felt the extramission theory of light was flawed, and that light came from a external luminous body. This became known as the intromission theory of light. Aristotle, however, did not have a fully developed view of light and—since much of the catalogue of his work disappeared from circulation until there was a revival of it in the Latin Medieval West—extramission theory dominated. 

It was Alhacen (965-1039 CE) from Basra, an Arab-Muslim scientist and mathematician, who changed everything. He provided an early, truly scientific approach, employing an inductive method of reasoning and questioning received knowledge, favoring experimentation and testing. Using pinhole projection, he was able to prove that light was from an external source.

He changed the conversation and paved the way for Newton, Huygens, Michelson, Morley, and Einstein. And yet, while the idea of the extramission of inner spiritual light was built squarely on extramission of physical light, intromission never ended the idea that there is an inner spiritual light and sight. 

But here’s the interesting thing: extramission prevailed in its many forms not because it came with an undeniable body of evidence (it didn’t), but because it is apparently our most intuitive way of thinking about light as human beings. According to one study of children, they think about sight and light as natural extramissionists. But more surprisingly, there are also a significant number of adults—including 50 percent of college students—who think in pre-scientific, extramissionist terms, even after receiving a college level education on the subject of light.

Extramission is intuitively much closer to our experience (we look out and see an object), while also being utterly wrong. But that is the great thing about science; it doesn’t care how you feel about something or that human beings have a long tradition of extramission. It seeks the truth and the data.

It seeks the light.

Photo credit: Amanda Dalbjörn on Unsplash