Prime Numbers
Though often described as numbers only divisible by one and themselves, I find it more useful to understand prime numbers as numbers that are only measurable by unity. Measurement requires calibration, so a more nuanced view might prefer to say prime numbers may only be calibrated by unity. Unity, the fundamental symmetry, defines the limit of continuity by preserving identity. So, prime numbers are calibrated by what defines the limit of continuity. In a sense, prime numbers illustrate the limits of continuity.