"The eye is the lamp of the body. So, if your eye is sound, your whole body will be full of light; but if your eye is not sound, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness!" (Mt 6:22-23)
What is Jesus talking about here?
In that time, people understood the eye to be both the organ and the process of understanding what is seen. They did not think that understanding occurred separate from the sensory organs. Rather, the senses acted through the organs themselves, and the organs brought meaning to what the senses perceived in the world.
Therefore, the condition of the eye naturally effected the entirety of perception. If one's physical perceptions are distorted, the one's entire reality is adversely impacted.
It is pretty easy to see how something like clinical depression fits into this. It isn't unscientific as it is just a different way of expressing what science rediscovered.