In many instances, it is of no interest to us how a particular machine is actually constructed--that is, we do not care whether a machine is made of silicon chips, wire, and steel, or whether it is constructed from tin cans, string, and matchsticks. All we want to know is what the behavioral relationship of the machine is to its defined environment. Such an abstract definition of a machine is necessary for purposes of analyzing what type of machine can achieve what end, and also to avoid dealing with the complexity of implementation at the same time as the analysis of abstract design.

Properties that are common to all machines are the processing of input and the production of output. The input may take several forms--a vending machine takes coins, an ATM takes a card, and a numeric lock takes a number punched on a keypad...