Today's digital computers are instantiations of Turing's theoretical breakthrough in conceiving the possibility of a man-made universal thinking machine—known nowadays as a 'Turing machine'. No physical mechanism can be intrinsically 'digital', Turing explained, since—examined closely enough—its possible states will vary without limit. But if most of these states can be profitably ignored, leaving only a limited set of relevant distinctions, then functionally the machine may be considered 'digital':[1]