This test was invented by Alan M. Turing (1912-1954) and first described in his 1950 article Computing machinery and intelligence (Mind, Vol. 59, No. 236, pp. 433-460).

How It Works

An interrogator is connected to one person and one machine via a terminal, and therefore can't see her counterparts. Her task is to find out which of the two candidates is the machine, and which is human only by asking them questions. If the interrogator cannot make a decision within a certain time (Turing proposed five minutes, but the exact amount of time is generally considered irrelevant), the machine is considered to be intelligent.

Criticism

This test has been subject to many different kinds of criticism, but it is the only one known - and as long as there is no definition for (human) intelligence, it will most probably remain so. The Turing Test is quite amazing in that it provides a test for something that today's science doesn't even have the remotest idea of!

The most important argument against the Turing Test, in my opinion, is that it only provides a test for human intelligence (see French, Robert M.: Subcognition and the Limits of the Turing Test). Even a person from a different culture might be considered a 'machine' (i.e., not intelligent) because of certain questions she wouldn't be able to answer, or would answer in an unexpected way. For example, asking about the side of the road you drive on would be answered in different ways, and there are more subtle differences people might not be aware of (a minor detail in everyday life you take for granted to be one way, while it is different in a different part of the world). Thus, the Turing Test shares its fate with early IQ tests the US Army used, and that immigrants usually failed because of their lack of knowledge of American culture.

For a funny take (and a nice illustration, at the same time) on the Turing Test, see Are You A Computer?.