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Let us now return to the analogy of the theoretical computing machines with
an infinite tape. It can be shown that a single special machine of that type can be
made to do the work of all. It could in fact be made to work as a model of any
other machine. The special machine may be called the universal machine; it
works in the following quite simple manner. When we have decided what
machine we wish to imitate we punch a description of it on the tape of the
universal machine. This description explains what the machine would do in
every configuration in which it might find itself. The universal machine has only
to keep looking at this description in order to find out what it should do at each
stage. Thus the complexity of the machine to be imitated is concentrated in the
tape and does not appear in the universal machine proper in any way.
If we take the properties of the universal machine in combination with the fact
that machine processes and rule of thumb processes are synonymous we may say
that the universal machine is one which, when supplied with the appropriate
instructions, can be made to do any rule of thumb process. This feature is paralleled
in digital computing machines such as the ACE. They are in fact practical versions of
the universal machine. There is a certain central pool ofelectronic equipment, and a
large memory. When any particular problem has to be handled the appropriate
instructions for the computing process involved are stored

Copeland, B. J. (2004-09-09). The Essential Turing (Page 383). Oxford University Press. Kindle Edition.