Ancient Philosophy
Since the earliest days of philosophy in ancient India and Greece, the true nature of time has exercised some the greatest minds in history.
Mythology
In ancient times, mythology and other traditional narratives were used to try and make sense of the universe we find ourselves in.
In Greek mythology, Khronos (or Chronus to the Romans) was the personification of time, not to be confused with Cronus, the Titan and father of Zeus. The Greeks had two different words for time: chronos refers to numeric or chronological time, while another word kairos refers to the more qualitative concept of the right or opportune moment. The figure of Khronos was typically portrayed as a wise old man with a long grey beard, similar to the later European folklore image of Old Father Time, although he was originally described in very early Greek mythological tales as serpentine in form, with three heads, of a man, a bull and a lion. A separate figure, Geras, was the Greek god of old age, usually depicted as a tiny shrivelled-up old man. The Horae or Hours were the goddesses of the seasons and the natural flow of time, generally portrayed as personifications of nature in its different seasonal aspects, and with the cycle of the seasons themselves symbolically described as the dance of the Horae.
Other mythologies had their own time-related gods, such as Heh the Egyptian deification of eternity or infinity, Zurvan the Zoroastrian god of infinite time (and the father of the twin spirits of good and evil), Elli the Norse god of old age, etc.
Wheel of Time
In ancient Indian philosophy, as expounded in early texts such as the Vedas of the late 2nd millennium BCE, the universe goes through repeated cycles of creation, destruction and rebirth (with each cycle lasting 4,320 million years according to some sources). This led to a cyclic view of time, the so-called “wheel of time” or Kalachakra, in which there are repeating ages over the infinite life of the universe. This was coupled with a belief in an endlessly repeated cycle of rebirths and reincarnations for individuals. The wheel of time concept is found in Hinduism and Buddhism, as well as in the beliefs of the ancient Greek Orphics and Pythagoreans, but also in other disparate religions and beliefs such those of the Maya, the Q’ero Indians of Peru and the Hopi Indians of Arizona.
The idea of time as consisting of endlessly repeated cycles is perhaps an unsurprising one given the observed repetitiveness of other natural phenomena, such as the day-and-night cycle, the motion of the tides, the monthly cycle of the Moon, the annual cycle of the seasons, etc. It does, however, seems to presuppose an overall linear ordering in some sort of “hypertime” of all the cycles, so that each cycle can be distinguished from its predecessors and successors because it occurs at a different point in hypertime.
Ancient Greece
The early Greek philosophers generally believed that the universe (and therefore time itself) was infinite with no beginning and no end. In the 5th Century BCE, the Sophist philosopher Antiphon asserted that time is not a reality (hypostasis), but a concept (noêma) or a measure (metron). Also in the 5th Century BCE, Parmenides saw time (as well as motion and many other everyday things that we take for granted) as nothing more than an illusion because, he argued, all change is impossible and illusory (time as an illusion is also a common theme in Buddhist thought). Parmenides, then, believed that reality was limited to what exists in the here and now, and the past and future are unreal and imaginary. His near-contemporary Heraclitus, on the other hand, firmly believed that the flow of time is real and the very essence of reality.
Zeno’s Paradoxes were devised at least partially to support Parmenides’ doctrine that change and plurality and the passage of time are merely illusory and lead to paradoxes and absurdity. In the best known of these, Achilles and the Tortoise, Achilles allows the tortoise a head start of, say, 100 metres in a footrace. After some finite time, Achilles will have run 100 metres, bringing him to the tortoise’s starting point, but during this time the slower tortoise has run a much shorter distance, say, 10 metres. It will then take Achilles some further time to run that distance, by which time the tortoise will have advanced yet further, etc, etc, so that whenever Achilles reaches somewhere the tortoise has been, he still has further to go. Because there are an infinite number of points Achilles must reach where the tortoise has already been, Zeno argues that he can never overtake the tortoise, and the tortoise must win the race. Indeed, another corollary of this paradox is that neither Achilles nor the tortoise can ever actually finish the race, as they are constantly having to cover an ever smaller distance, ad infinitum, and, as Zeno averred, “it is impossible to traverse an infinite number of things in a finite time”.
Plato, in the 4th Century BCE, believed that time was created by the Creator at the same instant as the heavens. But in an attempt to slightly be more scientific, Plato identified time with the period of motion of the heavenly bodies. Plata was also aware of the so-called “Great Year”, a complete cycle of the equinoxes around the ecliptic (effectively the return of the planets and the “fixed stars” to their original relative positions, a process that takes about 25,800 years). The Pythagoreans and some Stoic philosophers like Chrysippus saw the end of this cycle as the end of time iself, after which history would start to repeat itself all over again in an endless repetition.
Plato’s student Aristotle saw time as an attribute of movement, as something that does not exist on its own but is relative to the motions of things. He called time “the numeration of continuous movement” or “the number of change in respect of before and after”. Aristotle argued that time is essentially a measurement of change, and therefore cannot exist without some kind of succession or change, and also that it requires the presence of a soul capable of “numbering” the movement. Although he saw time as the measure of change, he stressed that it was not the same thing as change, because a change may occur faster or slower. Aristotle also believed that, although space was finite (with only some undefined void existing beyond the outermost sphere of the heavens), time was infinite, and that the universe has always existed and will always exist. Furthermore, he believed that time was continuous, not discrete or atomistic, in the same way as a line can be divided and sub-divided ad infinitum.
Aristotle was also the first to frame a commonly-mentioned paradox about the existence of time, recapitulated by St. Augustine several centuries later: if time essentially consists of two different kinds of non-existence (the past or the “no longer”, and the future or the “not yet”) separated by a nothing (the instantaneous and vanishing present or “now”), how then can we talk of time as actually existing at all?
The Dark Ages
Mithraism, a mystery religion influenced by ancient Zoroastrianism, and a strong competitor to Christianity in the early years CE, believed in a finite “Time of the Long Dimension” which repeated itself in cycles of 12,000 years, within the overall container of infinite time. In general terms, Zoroastrianism saw the world around us as a kind of battlefield between a bad god and a good one, and saw time as the duration of this battle.
The early Christian theologian St. Augustine (4th – 5th Century CE) probably thought more deeply about the nature of time than any philosopher since the ancient Greeks, but his deep thoughts remained inconclusive. Echoing the earlier comments of the Neo-Platonist Plotinus, St. Augustine famously encapsulated the experience of so many of us, when he observed: “What then is time? If no one asks me, I know; if I wish to explain it to one that asks, I know not”. He was only able to conclude that time was some kind of a “distention” of the mind which allows us to simultaneously grasp the past in memory, the present by attention, and the future by expectation. St. Augustine also adopted a subjective view of time, that time is nothing in reality but exists only in the mind’s apprehension of reality.
Middle Ages
Christian and Muslim philosophers tried their best to incorporate the ideas of Aristotle into their theology during the early Middle Ages, but they struggled mightily with his belief that time was infinite. Perhaps the first Christian writer to put forward a solid argument against the ancient Greek notion of an infinite past was the Alexandrian philosopher John Philoponus in the 6th Century. The doctrine was further developed and institutionalized by the Christian scholastics of the 11th-13th Century, including Thomas Aquinas and St. Bonaventure, as well as Muslim philosophers such as Al-Kindi and Al-Ghazali, and the Jewish philosophers Maimonides and Saadia Gaon.
Christianity and the other Abrahamic faiths, Islam and Judaism, believed in an all-powerful and infinite God (in contradistinction to everything else, which was therefore finite), and so medieval Christian, Muslim and Jewish philosophers and theologians developed the concept of the universe having a finite past with a definite beginning (the moment of its creation by God). Time, therefore, was necessarily finite in nature, a doctrine known as temporal finitism. The general Christian view is that time will come to a definite end with the end of the world, in the so-called “end-times” and the cataclysm of the Apocalypse.
The 13th Century Italian theologian Thomas Aquinas objected to Aristotle’s assumption of infinite time on the grounds that, although the universe could in theory have existed infinitely into the past, in fact it did not (it began with God’s creation of the Earth a finite time ago), warning that our imagination cannot always be trusted to tell us how things really are. The 13th Century philosophers Henry of Ghent and Giles of Rome made the rather fine distinction that the continuum of time does actually exist in reality and not just as a mind-dependent concept, but that it can only be distinguished into earlier and later parts by the mind.
Various versions of Christian creationism persist to this day, although not all are quite as literal as that of the medieval philosophers, or of the 17th Century bishop James Ussher, who famously concluded in the that the Earth was created by God on Sunday, 23rd October 4004BCE, at precisely 6pm! Young-Earth creationists still believe that God created the Earth, sometime within the last ten thousand years or so, over a period of 6 days, literally as described in the Genesis creation narrative. Others have even specified that He deliberately created it with the appearance of age, complete with fossils, rock strata, etc. Old-Earth creationists, on then other hand, have attempted to update their beliefs to take account of the scientifically proven age of the Earth (around 4.6 billion years) by claiming that the six days of creation in Genesis were not ordinary 24-hour days, but “God-days”, which may be the equivalent of millions or billions of years of human time. Still others claim that life was created relatively recently by God, but on a pre-existing old Earth.
In the 14th Century, the French mathematician Nicole Oresme was perhaps the first to try and put the study of time on a mathematical and scientific basis. He asked the question as to whether the celestial motions of the Sun, Moon and planets are commensurable, and so whether there is a “basic” time interval of which the day, month and year are all exact integer multiples. Oresme suggested that a creator of the universe might well have arranged things so, but his conclusion was that that no two celestial motions are actually commensurable, and so there is no such basic time interval.