From Augustine's 'if no one asks me, I know' to Newton's absolute time, Kant's form of intuition, and Einstein's relativity — the many faces of the most familiar mystery.
What is time — a real feature of the world, a form of the mind, or the very texture of our existence?
Time is at once the most intimate and the most elusive thing we know: we live wholly within it yet cannot say what it is. Aristotle tied it to motion and counting; Augustine turned inward and found it stretched across memory, attention and expectation. The scientific revolution set absolute, mathematical time against a relational view on which time is nothing but the order of events. Kant relocated it inside the mind as the form of all inner sense, while Bergson protested that lived duration cannot be reduced to the spatialized clock. Phenomenologists dissected the flowing 'now', logicians asked whether past, present and future are objective, and relativity dethroned simultaneity itself. Read together, these positions reveal how much of our worldview hangs on what we take time to be.
Pre-Socratic
For Heraclitus reality is inherently temporal: 'everything flows' and one cannot step twice into the same river. Time is the medium of ceaseless change, yet this flux is not chaos but is measured by the logos, the hidden order that unifies the passing of opposites.
On Nature (fragments).
Pre-Socratic (Eleatic)
Parmenides denies the reality of time and change altogether: what truly is neither came to be nor will pass away, but is a timeless, undivided 'now'. Past and future are illusions, for they would involve not-being; genuine reality is eternal presence, and temporal becoming belongs only to deceptive opinion.
On Nature (poem, fragments).
Ancient Greek (Peripatetic)
Time is 'the number of motion with respect to before and after' — not motion itself but its measurable, countable aspect. It presupposes change and a soul able to count the 'nows'; without a mind to number them, it is unclear whether time would fully exist. The 'now' both connects and divides past and future.
Physics, Book IV, chs. 10–14.
Christian (Patristic)
'What then is time? If no one asks me, I know; if I wish to explain it, I do not.' Past and future have no existence except in the soul: the past as memory, the future as expectation, the present as attention. Time is a 'distention' of the mind, and God, being eternal, stands wholly outside its flow, having created time with the world.
Confessions, Book XI.
Classical physics
Newton posits absolute time, which 'flows equably without relation to anything external', existing independently of events and motions. This mathematical time is the uniform framework within which all change is measured; the ordinary, 'relative' time of clocks is only its sensible and imperfect measure.
Mathematical Principles of Natural Philosophy (Principia, 1687), Scholium.
Rationalism
Against Newton, Leibniz holds that time is relational, not absolute: it is 'the order of successions', nothing over and above the ordered sequence of events. There is no empty time, for by the principle of sufficient reason God could have no ground to create the world at one moment of an indifferent time rather than another.
The Leibniz–Clarke Correspondence (1715–16).
German idealism / critical philosophy
Time is neither an absolute container nor a mere relation among things, but the a priori form of inner sense — the framework the mind imposes on all experience. It is 'empirically real' (valid for all appearances) yet 'transcendentally ideal' (not a feature of things in themselves); everything we experience is necessarily ordered temporally.
Critique of Pure Reason (1781), 'Transcendental Aesthetic'.
Process philosophy / vitalism
Bergson distinguishes lived time (durée), a continuous, qualitative flow interpenetrating past and present, from the spatialized time of science, which chops duration into homogeneous, countable instants. Real time is creative and irreversible, grasped by intuition rather than the intellect, and cannot be captured by clock or number.
Time and Free Will (1889); Creative Evolution (1907).
Phenomenology
Husserl analyses the internal structure of time-consciousness: each present moment carries a 'retention' of what has just passed and a 'protention' of what is anticipated, so the now is never a bare point but a living, extended flow. Objective time is constituted out of this deeper stream of inner temporal experience.
On the Phenomenology of the Consciousness of Internal Time (lectures 1905, publ. 1928).
British idealism / analytic metaphysics
McTaggart distinguishes the A-series (past, present, future) from the B-series (earlier-than, later-than) and argues that time requires the A-series, which is nonetheless self-contradictory, since every event would have to be past, present and future. He concludes, notoriously, that time is unreal.
'The Unreality of Time' (1908); The Nature of Existence (1927).
Phenomenology / existential ontology
Time is not primarily a series of nows but the very meaning of human existence: Dasein is temporal through and through, projecting toward its future, thrown from its past, and making present. Authentic temporality arises from confronting one's finitude — being-toward-death — and Heidegger seeks to interpret Being itself in terms of time.
Being and Time (1927).
Modern physics (relativity)
Special relativity abolishes absolute, universal time: simultaneity is relative to the observer, and time dilates for moving clocks. Time is woven with space into a single four-dimensional spacetime, its geometry shaped by gravity in general relativity. As a scientist, Einstein even wrote that the distinction between past, present and future is 'a stubbornly persistent illusion'.
'On the Electrodynamics of Moving Bodies' (1905); general relativity (1915).