French physicist Pierre Laplace came up with this demon to help show that all events are predetermined.

Laplace's demon has perfect knowledge of one moment of the universe -- in that moment he knows where every atom is and what forces are acting on it. He also has a very strong calculating device.

From the information he has on where things are at that point and what forces are acting on them, he should then be able to predict what will happen in the next moment, even if he has no direct knowledge of this next moment. And from this second moment, he should be able to calculate the one following that. And so on.

This was ment to show that at any one time (even without a demon), all the future events in the universe are determined by the past ones.

"We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at any given moment knew all of the forces that animate nature and the mutual positions of the beings that compose it, if this intellect were vast enough to submit the data to analysis, could condense into a single formula the movement of the greatest bodies of the universe and that of the lightest atom; for such an intellect nothing could be uncertain and the future just like the past would be present before its eyes."

-Marquis Pierre Simon de Laplace

Note that the demon was only there to help you get the idea, and is not necessary for determinism. Neither the observation nor the prediction of any part of the system is necessary to show predetermination to be the case.

This type of argument is one of the problems challenging the idea that we have free will.