"Logic 0" and "logic 1" represent binary digits (0 and 1) or Boolean logic conditions (true and false). Thinking in terms of logic 0 and logic 1 allow engineers to design circuits and logic gates at a high level of abstraction that is removed from implementation considerations. They can then "map" their high-level design onto different physical technologies using any two voltages as required; for example: ground (0 volts) and +5, ground and +3.3V, -5V and ground, -2.5V and +2.5V, etc. See
logical vs. physical and
binary.