Is Time an Illusion? Philosophers on What Time Really Is

Last updated: October 22, 2026

Whether time is an illusion depends on which part of time you mean, and philosophers have defended both answers in print. In 1908 the Cambridge philosopher J. M. E. McTaggart published “The Unreality of Time” in Mind and stated his position bluntly: “I believe that time is unreal.” Much of today’s debate, as the Stanford Encyclopedia of Philosophy entry on time lays it out, is narrower: does only the present exist (presentism), or do past, present and future exist alike (eternalism, the “block universe”)? Einstein, widely quoted as the man who called time an illusion, wrote something more specific. In a condolence letter dated 21 March 1955, according to the Christie’s catalogue that sold it, he said the separation between past, present and future is “an admittedly tenacious illusion”.

The meme compresses this whole literature into one panel. The caption reads: Me: “I’m going to have a normal conversation”. Also me: “Time is an illusion”. Below it, a grey alien in ornate robes stands glowing between dark trees, arms spread like a prophet. The joke is the moment a perfectly ordinary chat drifts into late-night cosmic pronouncements, and “time is an illusion” is the stock line for that drift. Douglas Adams gave the phrase a famous punchline in The Hitchhiker’s Guide to the Galaxy (1979): “Time is an illusion. Lunchtime doubly so.” The rest of its aura comes from Einstein, usually quoted in a form he did not quite write.

Augustine Knows What Time Is, Until Someone Asks

Augustine’s answer to “what is time?” is that he knows until someone asks him, and that time seems to live in the mind as memory, attention and expectation.

Book XI of the Confessions, written around 396 to 400, contains a much-quoted admission of defeat. In J. G. Pilkington’s translation (XI.14.17) Augustine asks what time is and replies: “If no one ask of me, I know”, and if he tries to explain it to someone who asks, he does not. The Latin is shorter still: si nemo ex me quaerat, scio. The past no longer exists, the future does not exist yet, and the present has no length, so what exactly are we measuring when we say a sermon lasted an hour? Augustine’s answer (XI.20.26) is that there are three times, all of them present: memory of things past, sight of things present and expectation of things future. A few chapters later (XI.26.33) he suggests time is nothing but a distentio, a stretching, and adds that it would be strange if it were not a stretching of the mind itself.

Augustine also refused the cheap way out. Asked what God was doing before he made heaven and earth, he reports someone’s joke answer, “He was preparing hell” for “those who pry into mysteries” (XI.12.14), and declines to use it. His serious reply (XI.13) is that the question has no content, because time was created with the world and there was no “before” in which God could have been idle. This puts Augustine on one side of a dispute that is still running: is time something the mind does to the world, or something the world does whether we watch or not? Aristotle, centuries earlier, had tied time to the world. In the Physics (IV.11, Hardie and Gaye translation) he defines it as “number of motion in respect of ‘before’ and ‘after’” and insists time does not exist without change, which takes us back to the older puzzle of flux in our post on Heraclitus and constant change.

McTaggart’s Proof That Time Is Unreal

McTaggart argued that time requires past, present and future, that these three properties contradict each other, and that time therefore cannot be real.

His paper split time into two series that philosophers still use. The A-series is the ordering of events as past, present or future, which shifts constantly: your birthday is future, then present, then past. The B-series orders events as earlier or later than one another, and that ordering never changes: the Battle of Hastings is earlier than the Moon landing, forever. McTaggart held that the B-series alone is not real time, because without the A-series nothing ever happens; events would just sit in a fixed order. The A-series, though, is incoherent. In his words, “Past, present, and future are incompatible determinations”, yet “every event has them all”. Every attempt to fix this by saying an event was future and will be past, he argued, smuggles the A-series back in and starts a vicious regress.

The argument has been picked apart ever since, and the SEP’s entry on McTaggart calls the best reconstruction of one key premise “highly contentious”. Few accept the conclusion, but the labels stuck. Philosophers who think the passage from future to past is a real feature of the world are now called A-theorists; those who think the earlier-later ordering is the whole story are B-theorists, and the SEP entry on time frames much of the modern debate this way. McTaggart himself was a British idealist who believed reality consists of persons, so “time is unreal” was one piece of a metaphysics in which only minds and their relations exist.

The Block Universe: Presentism vs Eternalism

The block universe is the view that past, present and future events all exist equally, as one four-dimensional whole, so the present is privileged only from where you happen to stand.

The SEP entry on time (section 6) sets out three main options. Presentism says “only present objects exist”: Socrates did exist, but does not now. Eternalism says past and future objects exist too, just elsewhere in time, the way Tokyo exists even though you are not there. The growing block theory splits the difference: past and present exist, the future does not, so “the correct ontology is always increasing in size”. Eternalism is the block universe, and it sounds a lot like Parmenides, who on the traditional reading denied that change is real at all, although the SEP entry on Parmenides warns that this reading is contested.

Physics is the reason the block universe is taken seriously. According to Einstein’s special theory of relativity, the SEP entry on time explains, there is “no fact of the matter” whether two distant events happen at the same time; observers moving differently disagree about it. That is awkward for presentism, which needs a single, universal “now” to say what exists. Steven Savitt’s SEP entry on being and becoming in modern physics reconstructs the argument made by Rietdijk and Putnam: if every event in your present is real, and observers moving past you have different presents, then the whole of spacetime, future included, must already be definite. Savitt also gives the replies. Howard Stein proposed defining becoming relative to each point’s past light cone, and the physicist Carlo Rovelli, who rejects both presentism and eternalism, argues that none of the usable definitions of “present” deserves “ontological weight”. The question is open, and presentists can resist by denying that no reference frame is privileged, a move the SEP notes does not conflict with any experiment.

What Einstein Actually Wrote to the Besso Family

Einstein’s 1955 letter called the distinction between past, present and future an illusion, did not call time itself unreal in the lines Christie’s quotes, and was written to console a grieving family.

Michele Besso, whom Einstein had befriended at musical evenings in Zurich as a student, died in March 1955. On 21 March Einstein wrote from Princeton, in German, to Besso’s son Vero and sister Bice Rusconi. The Christie’s catalogue renders the key lines: Besso “has again preceded me a little in parting from this strange world”, which “has no importance”, because for people who believe in physics the separation between past, present and future has “only the importance of an admittedly tenacious illusion”. The widely shared English version, “a stubbornly persistent illusion”, translates the same sentence. The slogan versions that circulate under Einstein’s name go further and swap “time” in for “past, present and future”. Einstein died four weeks later, on 18 April 1955.

The philosopher of physics Tim Maudlin has argued at IAI News that reading a private consolation as a doctrine of physics is a mistake, and that relativity, while it removes absolute simultaneity, keeps an objective earlier and later for any two events a signal could connect. Einstein was harder on philosophers in public. At the Société française de philosophie in Paris on 6 April 1922 he told Henri Bergson, as Evan Thompson recounts in Aeon, that “there is no time of the philosopher”, only psychological time and the time of physics.

Bergson’s Durée: The Time You Live and the Time Clocks Count

Bergson argued that lived time, which he called durée (duration), is a flowing whole that clock time can measure only after cutting it into identical units.

In his first book, the Essai sur les données immédiates de la conscience (1889, translated as Time and Free Will), Bergson distinguished two kinds of multiplicity. A flock of sheep can be counted because each sheep sits in its own place; the SEP entry on Bergson notes that, in the same way, “a calendar or a clock is a homogeneous form of time.” Conscious states work differently: they “permeate one another” and gradually gain a richer content, so the fifth minute of waiting for news differs in quality from the first. Bergson located freedom in that duration. When Einstein dismissed “the time of the philosopher” in 1922, this was, in effect, the time at stake, and Bergson published his reflections as Duration and Simultaneity. The SEP adds that Bergson wrote in 1924 that relativity “represents a very great advance”, and the SEP reads his quarrel as being about what physics leaves out of time, not as a rejection of the theory.

Martin Heidegger pushed the lived side further in Being and Time (1927). On Mark Wrathall’s reading in the SEP, human existence is temporal through and through: we act toward a future, carry a past and deal with the present at once, and the clock’s “succession of nows” is derived from that structure, although critics such as William Blattner argue the derivation fails. Heidegger’s finite time, shaped by the anticipation of death, gets a fuller treatment in our post on death as the basis of existential philosophy.

The Physics Definition of Time: 9,192,631,770 Cycles of Caesium Radiation

In physics, time is what clocks measure, and the official unit, the second, is defined by the radiation of caesium-133 atoms.

The International Bureau of Weights and Measures (BIPM) defines the second by fixing the frequency of a particular transition in the caesium-133 atom at 9,192,631,770 hertz. One second is that many oscillations. It is an operational definition, the physicist’s version of Aristotle’s “number of motion”: it tells you how to count time precisely, and it says nothing about whether time flows or why you remember yesterday and not tomorrow. Relativity adds that each clock measures its own time along its own path, which is why the SEP’s being-and-becoming entry discusses attempts to locate passage in this “proper time” rather than in a cosmic present.

Whether time is an illusion turns on which of three things you mean. Clock time is real enough for the BIPM to fix the second on it. A universal present that slides from past to future is under real pressure from relativity, which is why the block universe has serious defenders. The felt flow of time, Bergson’s duration and Augustine’s stretching of the mind, is a fact about experience whatever physics concludes about its cause. Which of them earns the name “time” is the metaphysician’s problem; our guide to metaphysics maps the neighbouring ones.

For the ancient side of the same puzzle, read The only thing that is constant is change: what Heraclitus actually said.