Time, like space, is defined relative to something else, which is considered universal. Space is bound by the idea of a meter, which is the distance a light can travel in 1 / 299,792,458 seconds. In the same manner, time is bounded by the idea of second, minute, hour, day, year. It is an arbitrary way to unify the spatio-temporal understanding of all the subjective conscious beings under an objective perspective, that is light. This idea is not attributed to me as much as it is for Albert Einstein when he thought of speed of light as the universal speed limit. Speed entails both distance and time; meters and seconds; space and time.

The equation shown above is Lorentz time dilation equation. Lorentz equation shows how time is altered with respect to other objects when the objects move in relativistic speeds. This means that if superman travels with half the speed of light around earth for a year, he would age only half a year and everyone on earth would age a whole year. The shocking part of the equation is if superman travels with speed of light for a year - excluding here breaking the law of E = mc2, since superman has super powers - superman would not age at all; however, everyone else would age a whole year. This is the definition of an instant, the instant of superman’s consciousness in this example lasted for a year. No aging, no change, not a single thought is produced to alter the consciousness of superman.

To conclude, this idea of an instant and how consciousness is stable only in an instant is important for arguments against the unity of consciousness, and an essential idea to communicate thought experiments like Theseus ship or splitting the brain into two bodies.