Alan Turing postulated that a machine could be created that would be indistinguishable from a human when presented in a game known as the Turing test. However, a better definition of machine is necessary.

The machine Turing imagines is a digital computer. A digital computer is one that can store information and carry out specific tasks which are regulated by a control element. The machines and analytical engines of Turing's time and earlier were mainly mechanical. The computation had various positions in which the wheels and gears comprising the machine would be in. It is said that these digital computers could have discrete states or varying positions. Therefore a digital computer could emulate a discrete state machine. A discrete state machine with infinite memory could have infinitely many states. With enough states the discrete state machine could imitate humans. Allow me to digress.

Imagine an adding machine and a separate subtracting machine, multiplying machine, and dividing machine. Each separate machine is essentially a digital computer. The machine would store input. It's control element would regulate the appropriate arithmetic action. Imagine a discrete state machine that had memory for four discrete states: that of an adder, subtracter, multiplier, and divider. Supposing the discrete state machine had enough memory available, it could take on more and more discrete states and therefore mimic more machines and possibly humans.

At the end of Turing's essay titled "Computing Machinery and Intelligence" he concludes that intelligent machines are possible, provided enough memory is available. Also, he states that more experimentation is necessary to discover an intelligent machine.