It can be demonstrated that if a sufficiently large number of linear
threshold elements is used, with the outputs of some being the inputs
of others, then a final output can be produced which is any desired
logical function of the inputs. The difficulty in such a network lies
in the fact that we are no longer provided with a knowledge of the
correct output for each element, but only for the final output. If the
final output is incorrect there is no obvious way to determine which
sets of weights should be altered.
As a result of considerable study and experimentation at Aeronutronic,
a network model has been evolved which, it is felt, will get around
these difficulties. It consists of four basic features which will now
be described.