If we think of the history of computer technology, there have been different stages of development. Charles Babbage invented the first computer - the differential engine - using brass gears and cogs. Later, vacuum tubes, transistors and ultimately silicon chips were employed in the development of faster and more efficient hardware processing.

However, whatever material is used to make up the hardware of a computer, the calculations performed on the machine will be transferable. In other words, someone could write a program using two completely different types of computer which use different sorts of hardware to run the same program. In this sense, the program is said to be "multiply realisable" in that any number of computers may be used to realise the program.

If we think of mental states in this way - that is, as "multiply realisable" - then it is possible that there may one day be a machine that can "think", or be artificially intelligent. In this way, the process of thinking would be comparable to a computer program that could run on different types of machine.