Understanding these verses requires that we shift our minds from modern medical understanding to an ancient conception. The modern world knows that the eye is a receptor. Light comes from an external source and enters into the eye. In the ancient world, the eye was an active source of light. The light that “sees” came from the eye and extended outward. It is in this way that the eye is the light of the body. The Greek text is even clearer, for it states that the eye is the lamp of the body.
The second aspect of this teaching is the contrast between light/dark as a contrast between good/bad. This symbolic contrast is sufficiently pervasive in our own culture that we understand it quickly. It is the combination of this metaphorical usage of light and darkness with the idea that the eye projects light from inside out that allows us to understand how this teaching was understood.
The light or lamp of our body projects light. The optimum condition is when that eye is “single.” The Greek might also be translated as “sound.” The particular contextual meaning for “sound” is one of wholeness, completeness, or without blemish (Robert Guelich. A Foundation for Understanding the Sermon on the Mount. Word Publishing, Dallas. 1982, p. 330). Thus when the lamp is pure or perfect, it can project a soul full of light.
The contrast is given in the KJV as “evil,” but should be read as the opposite of “single/sound.” Thus we have one case where the light if full because the eye us sound and another where there is darkness because the light is unsound. It is quite probable that there is a naturalistic picture that was well known standing behind these images. The ancient world was well acquainted with lamplight at night, and would be well aware of the amount of light given off, and particularly the quality of the light. If there were an unsound fuel source, the light would not be as consistent, or possibly as bright. Thus the soundness of the source was directly seen in the quality of the light given off.
It is in this context that we see the darkness in the soul. With the unsound eye or lamp, the evidence of the dimming or inconsistent light was an impurity of the source. This teaching serves to underline many of the teachings of the earlier part of the Sermon where the importance of the internal spiritual change is considered to be the most important. The light emanating form one’s eye was evidence of that internal quality.
Book of Mormon Context: We have no evidence for the cultural conception of the light as lamp or receptor. We do not know how a Mesoamerican audience would have understood this reference.
Textual: There is no change from the Matthean text.