Where to draw the line between analog and digital?

[Tom]

It is true that a theoretic digital signal is discontinuous, but the output of a digital gate is still a continuous voltage. Digital signals show up as continuous traces on an oscilloscope screen. So yes, an abstract digital process involves discrete ones and zeros, but digital circuitry is really an analog approximation of an abstact construct.

If digital circuitry was ideal, you could speed the digital clock up to arbitrarily fast speeds. In fact, the maximum clock frequency depends on how well the analog approximation matches the digital theory.