According to Bejan—who reviewed previous studies in a range of fields on time, vision, cognition, and mental processing to reach his conclusion—time as we experience it represents perceived changes in mental stimuli. It’s related to what we see. As physical mental-image processing time and the rapidity of images we take in changes, so does our perception of time. And in some sense, each of us has our own “mind time” unrelated to the passing of hours, days, and years on clocks and calendars, which is affected by the amount of rest we get and other factors. Bejan is the first person to look at time’s passage through this particular lens, he tells Quartz, but his conclusions rest on findings by other scientists who have studied physical and mental process related to the passage of time.

These changes in stimuli give us a sense of time’s passage. He writes:

The present is different from the past because the mental viewing has changed, not because somebody’s clock rings. The “clock time” that unites all the live flow systems, animate and inanimate, is measurable. The day-night period lasts 24 hours on all watches, wall clocks and bell towers. Yet, physical time is not mind time. The time that you perceive is not the same as the time perceived by another.

Time is happening in the mind’s eye. It is related to the number of mental images the brain encounters and organizes and the state of our brains as we age. When we get older, the rate at which changes in mental images are perceived decreases because of several transforming physical features, including vision, brain complexity, and later in life, degradation of the pathways that transmit information. And this shift in image processing leads to the sense of time speeding up.