*Two contrasting viewpoints* on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time.Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line. The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events (such as, change of conditions within an ever-present, allowing an endless 'succession' of changes). This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.
Ray Cummings, an early writer of science fiction, wrote in 1922, "Time... is what keeps everything from happening at once", a sentence repeated by scientists such as C. J. Overbeck, and John Archibald Wheeler.
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