An Experiment with Time is a book by the British soldier, aeronautical engineer and philosopher J. W. Dunne (1875–1949) about his precognitive dreams and a theory of time which he later called "Serialism". First published in March 1927, the book was widely read. Although never accepted by mainstream scientists or philosophers, it has influenced imaginative literature ever since. Dunne published four sequels: The Serial Universe (1934), The New Immortality (1938), Nothing Dies (1940) and Intrusions? (1955).

An Experiment with Time discusses two main topics.

The first half of the book describes a number of precognitive dreams, most of which Dunne himself had experienced. His key conclusion was that such precognitive visions foresee future personal experiences by the dreamer and not mere general events.

The second half develops a theory to try to explain them. Dunne's starting point is the observation that the moment of "now" is not described by science. Contemporary science described physical time as a fourth dimension and Dunne's argument led to an endless sequence of higher dimensions of time to measure our passage through the dimension below. Accompanying each level was a higher level of consciousness. At the end of the chain was a supreme ultimate observer.

According to Dunne, our wakeful attention prevents us from seeing beyond the present moment, whilst when dreaming that attention fades and we gain the ability to recall more of our timeline. This allows fragments of our future to appear in pre-cognitive dreams, mixed in with fragments or memories of our past. Other consequences include the phenomenon known as deja vu and the existence of life after death.

Following a discussion of brain function in which Dunne expounds mind-brain parallelism and highlights the problem of subjective experience, he gives anecdotal accounts of precognitive dreams which, for the most part, he himself had experienced.

The first he records occurred in 1898, in which he dreamed of his watch stopping at an exact time before waking up and finding that it had in fact done so. Later dreams appeared to foretell several major disasters; a volcanic eruption in Martinique, a factory fire in Paris, and the derailing of the Flying Scotsman express train from the embankment approaching the Forth Railway Bridge in Scotland.

Dunne tells how he sought to make sense of these dreams, coming slowly to the conclusion that they foresaw events from his own future, such as reading a newspaper account of a disaster rather than foreseeing the disaster itself. In order to try and prove this to his satisfaction, he developed the experiment which gives the book its title. He kept a notepad by his bedside and wrote down details of any dreams immediately on waking, then later went back and compared them to subsequent events in his life. He also persuaded some friends to try the same experiment, as well as experimenting on himself with waking reveries approaching a hypnagogic state.