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There are a wide variety of short answers to the question "What is time?" Plato said time is the circular motion of the heavens. Aristotle said it's not motion but the measure of motion.St.Augustine said time is nothing in reality but exists only in the mind's apprehension of that reality. Henry of Ghent and Giles of Rome both said time exists in reality as a mind-independent continuum, but is distinguished into earlier and later parts only by the mind. Kant said time is a form that the mind projects upon the external things-in-themselves. A modern definition says time is the dimension of causality. Let's explore some of these answers.
Aristotle provided an early, careful answer to the question "What is time?" when he said time is the "number of movement in respect of the before and after, and is continuous.... In respect of size there is no minimum; for every line is divided ad infinitum. Hence it is so with time." [Physics, 220a] In these passages, Aristotle argues that time is neither the circular motion of the heavens (Plato's view) nor any other motion. He believes time is something by which we measure motion. Time is like a line, he says; and it is continuous rather than discrete. The line he had in mind was a circle [223b], a structure that has no beginning or end point and so is endless in both directions. Saint Augustine objected to Aristotle's belief that time is circular, insisting that human experience is a one-way journey from Genesis to Judgment, regardless of any recurring patterns or cycles in nature. Thomas Aquinas agreed. In 1687, Newton captured some of this viewpoint when he represented time by using a line rather than a circle. Aristotle argued that we cannot conceive of a first time because for any such time we could conceive of a time before that. Thomas Aquinas criticized the assumption that something doesn't exist if humans can't conceive it.
Aristotle raised the issue of whether time exists without consciousness: "Whether, if soul did not exist, time would exist or not, is a question that may fairly be asked; for if there cannot be some one to count there cannot be anything that can be counted..." [223a] He doesn't answer his own question because, he says, it depends on whether time is the conscious numbering of movement or instead is just the capability of movement's being numbered were consciousness to exist. Aristotle's distinction foreshadows the modern distinction between psychological time and physical time.
Physical time is public time. Psychological time is private time. We are referring to psychological time when we say that time passes slowly while we are waiting for the water to boil on the stove. We are referring to physical time when we speak of the time that a clock measures, or when we define speed to be the rate of change of position with respect to time. Psychological time is best understood as being consciousness of physical time. Psychological time stops when consciousness does, but physical time does not. Physical time is more basic for helping us understand our shared experiences in the world. It is more useful than psychological time for doing science. In the 11th century, the Persian philosopher Avicenna doubted the existence of physical time, arguing that time exists only in the mind due to memory and expectation, but Duns Scotus in the 13th century recognized both physical and psychological time.
In the 17th century, the English physicist Isaac Barrow rejected Aristotle's linkage between time and change, or between instants and events, by saying that time is something which exists independently of motion and which existed even before God's creation. Barrow's student, Newton, agreed. Newton added that motion (your speed, for example) is relative to the reference frame you are analyzing it from, but that there is a special reference frame in which real time (absolute time) is the measured time. Newton also argued very specifically that time and space are substances that provide an infinitely large container for all events; this container is the absolute reference frame. Gottfried Leibniz objected. He argued that time is not a substantial entity existing independently of those events. Leibniz insisted that Aristotle and Newton had overemphasized the relationship between time and duration, and underemphasized the fact that time ultimately involves order as well. Time is an ordering of changes, the overall ordering of all non-simultaneous events. Leibniz added that this order is also a "something" as Newton had been insisting, but it is an ideal entity, not a concrete one as Newton was mistakenly supposing it to be. Trees and stars are concrete entities. Triangles, numbers, and relations are ideal entities.
In the 18th century, Immanuel Kant said time and space are forms that the mind projects upon the external things-in-themselves. He spoke of our mind structuring our perceptions so that space always has a Euclidean geometry, and time has the structure of the infinite mathematical line. Kant's idea that time is a form of apprehending phenomena is probably best taken as suggesting that we have no direct perception of time but only the ability to experience things and events in time. Some historians distinguish perceptual space from physical space and say that Kant was right about perceptual space. It's difficult, though, to get a clear concept of perceptual space. If physical space and perceptual space are the same thing, then Kant is claiming we know a priori that physical space is Euclidean. With the discovery of non-Euclidean geometries in the 1820s, and with increased doubt about the reliability of Kant's method of transcendental proof, the view that truths about space and time are apriori truths began to lose favor.
In 1924,Hans Reichenbach defined time order in terms of possible cause. Event A happens before event B if A could have caused B but B couldn't have caused A. This was the first causal theory of time. Its usefulness depends on a clarification of the notorious notions of causality and possibility.
One proper, but indirect, way to answer the question "What is physical time?" is to declare that it is whatever the time variable t is denoting in the best-confirmed and most fundamental theories of current physics. Many philosophers complain that this answer is incomplete because, although philosophical theories of time should be informed by what science requires of time, they should progress beyond.
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