One of the greatest thinkers in history and natural sciences was the Greek philosopher Aristotle (384-322 BC), a pupil of Plato and subsequently the teacher of Alexander the Great. His work encompasses the natural sciences and philosophy including logic, botany-zoology, physics, ethics, politics, metaphysics, et al. A large part of his work, however, is devoted to the study of the natural world. In the following article we shall study the views expressed by the philosopher regarding the nature of time, a natural quantity that mankind has tried to determine since very ancient times. Specifically, we shall explore the meaning of now (νυν), the point of reference in defining time. Next, the relation of time to movement will be examined, leading to the definition of time. Last we shall refer to man’s perception of time.
The Now (Νυν)
Man’s perception of time requires determining the concepts of “past” and “future”, based on which our conscience organizes reality. In this manner man is in a position to determine when an event took place and consequently organize his life.
Aristotle, however, believed that the perception of the past and future are defined by a third dimension which constitutes the present and which Aristotle names “now”, “νυν”.
Aristotle accepted that the now constitutes a mediocrity (μεσότητα) which determines the end of past time and the beginning of the future (Aristotle, Physics [Φυσικής Ακροάσεως], 251b, 25-29). It is in fact characteristic that Aristotle’s views influenced science for many centuries to come. Even the structure of space-time accepted by Newton and Galileo was made up of a present expanding in space, separating the past from the future (Luminet 2006, p. 49). Therefore, our consciousness uses reference points in order to perceive time and the various events taking place.
John Philoponus, an eminent Byzantine natural scientist, philosopher and commentator of the works of Aristotle, believed that the now is a creative cause of time (Philoponus, Against Proclus [Κατά Πρόκλου], 727, 21), since it is the factor by which time is determined. He also defines it as an instant event (Philoponus, On Aristotle’s Categories [Εις Κατηγορίας], 46, 19), which means it has no duration, while barely being consciously perceived.
Aristotle, however, adds that successive “nows” have no real existence, but are ideal breaking points in the flow of time. They are instantaneous and are nothing but a “convention” for us to understand the difference between the past and the future.
Moreover, for Aristotle to show that all the “nows” do not make up time, he likens them to points that constitute a line. Therefore, just as the line (length) is not made up of moments (Aristotle, Physics 241a, 2-4) but of a continuous flow, likewise time is not made up of “nows” but of a constant flow which is perceived by human physiology as a flow towards the future, hence it is described as fluid (ροώδης) (Aristotle, Physics 241a, 2-4). Nowadays, science talks of the “arrows of time” which include the psychological aspect that precisely consists of understanding the flow of time from the past to the future.
One concludes from the above that each “now” is instantaneous and switches directly to the past, allowing consciousness to perceive the flow of time. Therefore, K.D. Georgoulis rightly argues that the “nows” constitute a “juxtaposition of infinitesimal moments that do not make up time which has a constant and uninterrupted flow.” (Georgoulis 2000, p. 296).
Time and Movement
The views of Aristotle on the relationship between time and movement start from acknowledging that while time and movement are synchronized they are not however identical.
Specifically, Aristotle writes that movement is a feature located only in a particular object and can be slower or faster on occasions. Therefore, it is determined only by the movement made by a specific body. Time, on the contrary, is uniform for all beings, irrespective of their movement, “Time itself is the same everywhere” (Aristotle, Physics, 223b 12). So if time was to be identical to movement, each being would have a different perception, consequently there would be more than one time. Nevertheless, Modern Physics has proved by the theory of Special Relativity that as speed increases (approaching, in fact, the speed of light), time is annihilated (Serway et al., p. 13), a phenomenon known as time dilation.
Based therefore, on the theory of Special Relativity, every observer has his/her own notion of the flow of time, a fact which negates Aristotelian views on the uniformity of time.
Aristotle, however, further argues that the uniqueness of time results from the existence of a kind of matter and therefore of a world and time measured by a circular motion. He uses circular movement because the circle is considered a perfect shape, since all its points have an equal distance from the centre. Consequently, the uniformity of the circle is identical to the uniformity of time. Aristotle notes accordingly that “to say that things taking place are a circle, is like saying that a circle of time exists” (Aristotle, Physics, 223b, 36-37).
Next, the philosopher examines the flow of time by relying on the possibility of time being divided into instantaneous moments. For this reason, he provides two points on the time flow called before (πρότερον) (t1) and after (ύστερον) (t2) and which can be counted because of their distance from the now. In reality these two points constitute two nows, the one separate from the other, interrupted by an interval. During this interval (t1, t2) the soul-observer records a movement (flow) that is perceived as time (Aristotle Physics,219a, 22-25). As a result, the perception of time on the part of the soul requires defining the concepts of before and after, between which movement takes place (Philoponus, Commentary on Aristotle’s Physics [Εις τα Φυσικά], 720, 26). Before and after are really two nows that naturally do not have an objective existence, since they play the part of reference points by which movement is perceived consciously; i.e. they determine a period in the time flow, during which some movement or activity has taken place. If, for example, with the help of a watch, we accept 10 in the morning as being before and 11 in the morning as after, our planet has covered part of its orbit round the sun while simultaneously revolving on its axis. We consciously perceive this movement as time, which serves both in regulating our daily activities and in creating calendars, thanks to the recording of the earth’s movements round its own axis and round the sun (with a gradient of 23.5 degrees that contributes to creating the seasons – the phenomenon of the obliquity of the ecliptic).
As becomes obvious from the above, according to Aristotelian thought, time is an intellectual activity requiring the presence of an observer who records movement.
Man’s perception of time
The eminent Byzantine scholar John Philoponus attempted to answer the question as to whether the absence of an observer recording movement automatically results in the absence of time as well. The Aristotelian system on time consists of three factors:
1) The one to be counted (αριθμούμενο) (movement [κίνηση]).
2) The soul [ψυχή, anima] (observer) who measures movement.
3) The result of measuring the movement (time). In the absence therefore of the soul (observer) there will be no measurement and consequently no time, the result, that is, of measurement (Aristotle Physics,223a, 24-27).
This is the reason why Philoponus argues that when the soul is absent, time is absent also (ψυχής αναιρεθείσης, συναναιρείται και ο χρόνος) (Philoponus, Commentary on Aristotle’s Physics, 775, 12).
The use of the term soul in this case does not have a metaphysical meaning since it is directly linked to the existence of time, that is the result of observing and measuring movement. Consequently Philoponus separates the natural occurrence of movement from the mental process of its being counted as, of course, Aristotle had done earlier on.
Can time be mentioned however, without the existence of observers? Aristotle answers this question by saying that in reality “if the soul does not exist, time would not have been possible. But either this, or time would be a being (ον) such as would have been the case if movement existed without a soul. Then before or after would be but movement and they would make up time by being countable” (Aristotle, Physics, 223a, 27-36). As expected, before the appearance of man and possibly other beings in the universe, no observer is found to record the movements of celestial bodies, so naturally time should not exist as a mental activity. On the contrary, there is constant movement apparent both in the structural components of cosmic matter and in planetary systems that are orbiting the stars (Kalachanis et al. 2012, pp. 14-15). By necessity, therefore, before the appearance of man, Earth continued to move both round the sun and itself, the only difference being that this had not been recorded by observers. It is then obvious that without some form of mental activity there is only movement, without, of course, the question of time being raised (Kalachanis 2011, p. 124).
Time and human physiology
Nature has endowed man with such spiritual and mental abilities as to help him considerably develop a civilization which, to a great degree, is based on determining units for measuring time. In reality, however, it is man’s own physiology that is “programmed to understand time”.
The human brain, that miracle of biological evolution, has its own systems by which it understands time. The function of the hypothalamus is crucial to this understanding of time, because it regulates the function of the endocrine system (Kandel et al. 2005).
Thanks to the hormones secreted by the hypothalamus, our body maintains a “biological clock” that is very useful in programming its functions. A characteristic example is the function of sleep which is determined by the secretion of melatonine, a hormone that regulates the levels of our sleep and alertness. The functioning of the senses is however definitely required in functions such as sleep, for the length of both day and night to be perceived.
Apart from sleep, another aspect of the “biological clock” concept is the monthly cycle of women (the period) which lasts about 28 days and is in fact a very well regulated system that “perceives” the passage of time.
It appears, therefore, that our organism understands time in its own manner. Of course, even before the appearance of man, there were beings such as dinosaurs whose endocrine system obviously understood time flow in its own way.
The above study showed that Aristotle believes man’s perception of time requires determining points of reference, amongst which our consciousness understands movement. Therefore, time is essentially the measurement of movement realized by our consciousness which perceives time as a flow. Furthermore, however, it becomes clear that the perception of time is not only an intellectual activity but is also an important factor in regulating our “biological clock”.
PhD in Philosophy, University of Athens
Research Associate, Physics Department of the University of Athens