  In his investigation, Turing introduced the idea of an ‘oracle’ capable of performing, as if by magic, an uncomputable operation. Turing's oracle cannot be considered as some ‘black box’ component of a new class of machines, to be put on a par with the primitive operations of reading single symbols, as has been suggested by (Copeland 1998). An oracle is infinitely more powerful than anything a modern computer can do, and nothing like an elementary component of a computer. Turing defined ‘oracle-machines’ as Turing machines with an additional configuration in which they ‘call the oracle’ so as to take an uncomputable step. But these oracle-machines are not purely mechanical. They are only partially mechanical, like Turing's choice-machines. Indeed the whole point of the oracle-machine is to explore the realm of what cannot be done by purely mechanical processes. Turing emphasised (Turing 1939, p. 173):