Mind, Body, World- Foundations of Cognitive Science, 2013a

How does a Turing machine generate answers to the written questions? A
Turing machine consists of a machine head whose actions are governed by a set
of instructions called the machine table. The machine head will also be in one of
a set of possible physical configurations called machine states. The machine head
reads a symbol on the tape. This symbol, in combination with the current machine
state, determines which machine table instruction to execute next. An instruction
might tell the machine head to write a symbol, or to move one cell to the left or
the right along the tickertape. The instruction will also change the machine head’s
machine state.
A Turing machine does not answer questions instantly. Instead, it takes its
time, moving back and forth along the tape, reading and writing symbols as it
works. A long sequence of actions might be observed and recorded, such as “First
the machine head moves four cells to the right. Then it stops, and replaces the 1 on
the tape with a 0. Then it moves three cells to the left.”
The record of the observed Turing machine behaviours would tell us a great
deal about its design. Descriptions such as “When given Question A, the machine
generated Answer X” would provide information about the input-output mapping
that the Turing machine was designed to achieve. If we were also able to watch
changes in machine states, more detailed observations would be possible, such as
“If the machine head is in State 1 and reads a ‘1’ on the tape, then it moves one cell
left and adopts State 6.” Such observations would provide information about the
machine table that was designed for this particular device’s machine head.
Not all Turing machine behaviours occur by design; some behaviours are artifacts.
Artifacts occur because of the device’s design but are not explicitly part of the
design (Pylyshyn, 1980, 1984). They are unintentional consequences of the designed
procedure.
For instance, the Turing machine takes time to add two numbers together;
the time taken will vary from question to question. The amount of time taken to
answer a question is a consequence of the machine table, but is not intentionally
designed into it. The time taken is an artifact because Turing machines are designed
to answer questions (e.g., “What is the sum of these two integers?”); they are not
explicitly designed to answer questions in a particular amount of time.
Similarly, as the Turing machine works, the ticker tape adopts various intermediate
states. That is, during processing the ticker tape will contain symbols that are
neither the original question nor its eventual answer. Answering a particular question
will produce a sequence of intermediate tape states; the sequence produced
will also vary from question to question. Again, the sequence of symbol states is an
artifact. The Turing machine is not designed to produce a particular sequence of
intermediate states; it is simply designed to answer a particular question.
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