Your eyes are far more than your windows to the outside world. They are the movie cameras that project information to the cerebral cortex, the brain's hardworking visual processor.
In absolute silence and utter darkness, the information is translated inside your skull at amazing speeds. It's a complex operation: in a split second, shadows, movement, and shape are first separated and then knit back together again by a workforce of millions of neurons.
In the final stage, the subconscious brain must decide just how much of the imagery it will make available to your conscious mind. The brain only has a limited processing capacity, so these edits are an essential element to the process. Since the revisions happen outside of your awareness, the concious brain is forced to play the part of moviegoer rather than director.
How does your subconscious decide what to keep and what to leave on the metaphorical cutting room floor? Scientists are still in the dark (couldn't resist).
But it is understood that your eyes, and by that I mean your pupils in particular, give us a window into your brain's ability to focus. We know, for example, that there is a direct correlation between the dilation of pupils and mental effort. The larger the pupils, the more concentration on display.
Many, many tests have proved this relationship. When I'm working hard on a math problem, for instance, my pupils become quite large. When I give up on the problem, my pupils immediately shrink, as if to say, "Hey, that's all folks."
Pupil dilation also occurs when you looks at someone you find attractive. But interestingly, the reverse is also true.
Not so long ago, scientists ran an experiment wherein they showed men photographs of swimsuit models. The first photo was genuine, but in the second picture, the models' pupils had been photoshopped to look larger. Sure enough, 90% of the men preferred the doctored photographs.
This is hardly news. The scientists could've saved a lot of time had they been able to talk to seventeenth century women. A common beauty trick at the time was to cut a sprig of the belladonna plant and sniff it from time to time which—you guessed it—dilates the eyes. Apparantly this trick was quite effective. (Hopefully it was worth it; belladonna is also quite poisonous.)
So how do restauranteurs take advantage of this? They dim the lights at night, ostensibly to create "ambiance". The lower light levels make the pupil dilate, which suddenly makes the person across from you far more desirable. This generally leads couples to linger in the restaurant longer, which naturally leads to another glass of wine, or a piece of cheesecake. And so with a flick of a switch, everybody is happy, especially the restaurant owner.
The eyes might not be the windows to the soul, as many a poet has suggested. But you could certainly make a strong case that the eyes, in their ability to reflect both inward and outward, help define both what we see and who we are. That alone is worth the price of admission.