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Time and space

A traditional realist position in ontology is that time and space have existence apart from the human mind. Idealists deny or doubt the existence of objects independent of the mind. Some anti-realists whose ontological position is that objects outside the mind do exist, nevertheless doubt the independent existence of time and space.

Kant, in the Critique of Pure Reason, described time as an a priori notion that, together with other a priori notions such as space, allows us to comprehend sense experience. Kant denies that either space or time are substance, entities in themselves, or learned by experience; he holds rather that both are elements of a systematic framework we use to structure our experience. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantitatively compare the interval between (or duration of) events. Although space and time are held to be transcendentally ideal in this sense, they are also empirically real, i.e. not mere illusions.

Idealist writers such as J. M. E. McTaggart in The Unreality of Time have argued that time is an illusion.

As well as differing about the reality of time as a whole, metaphysical theories of time can differ in their ascriptions of reality to the past, present and future separately.

Time, and the related concepts of process and evolution are central to the system-building metaphysics of A. N. Whitehead and Charles Hartshorne.