theoretical—an exercise in logic.
He was interested in reducing a
numbers task to its simplest, most
basic, automatic form.
The a-machine
To help envisage the situation,
Turing conceived a hypothetical
machine. The “a-machine” (“a” for
automatic) was a long paper tape
divided into squares, with one
number, letter, or symbol in each
square, and a read/print tape head.
With instructions in the form of a
table of rules, the tape head reads
the symbol of the square it sees,
and alters it by erasing and printing
another, or leaves it alone, as per
the rules. It then moves to one
square either to the left or right,
and repeats the procedure. Each
time there is a different overall
configuration of the machine,
with a new sequence of symbols.
The whole process can be
compared to the number-sorting
algorithm above. This algorithm is
constructed for one particular task.
Similarly, Turing envisaged a range
of machines, each with a set of
instructions or rules for a particular
undertaking. He added, “We have
only to regard the rules as being
capable of being taken out and
exchanged for others and we have
something very akin to a universal
computing machine.”
Now known as the Universal
Turing Machine (UTM), this device
had an infinite store (memory)
containing both instructions and
data. The UTM could therefore
simulate any Turing machine. What
Turing called changing the rules
would now be called programming.
In this way, Turing first introduced
the concept of the programmable
computer, adaptable for many
tasks, with input, processing of
information, and output. ■A PARADIGM SHIFT 253
A computer would deserve
to be called intelligent if it
could deceive a human into
believing that it was human.
Alan TuringA Turing machine is a mathematical model of a computer.
The head reads a number on the infinitely long tape, writes a
new number on it, and moves left or right according to rules
contained in the action table. The state register keeps track
of the changes and feeds this input back into the action table.
10 - 1110011 0 - 100Read/print tape headState register Action tableSee also: Donald Michie 286–91 ■ Yuri Manin 317
Alan Turing
Born in London in 1912, Turing
showed a prodigious talent for
mathematics at school. He
earned a first class degree in
mathematics from Kings
College, Cambridge, in 1934,
and worked on probability
theory. From 1936 to 1938, he
studied at Princeton University
in the US, where he proposed
his theories about a generalized
computing machine.
During World War II, Turing
designed and helped build a
fully functioning computer
known as the “Bombe” to
crack German codes made by
the so-called Enigma machine.
Turing was also interested in
quantum theory, and shapes
and patterns in biology. In
1945, he moved to the National
Physics Laboratory in London,
then to Manchester University
to work on computer projects.
In 1952, he was tried for
homosexual acts (then illegal),
and two years later died from
cyanide poisoning—it seems
likely this was by suicide
rather than by accident.
In 2013, Turing was granted
a posthumous pardon.Key work1939 Report on the Applications
of Probability to Cryptopgraphy