 This comparison illustrates an important distinction between actual motion from one part of space to another and the mere tendency to motion. Just as all the parts of the wine in the vat tend to move in a straight line toward the holes at the bottom of the vat, so too light is simply a tendency the smallest parts of matter between our eyes and the sun (or any other luminous object) have to move in a straight line toward our eye. The space between our eyes and any luminous object is so crammed that the smallest parts of matter cannot actually travel from the luminous object to our eye. Instead, their “action” consists in the tendency they have to move towards our eyes. This tendency exerts pressure on our eye, and this pressure, when communicated to the brain via the nerves, produces the sensation of light in the mind.