Early Modern Philosophy
After a long lull, the nature of time once again became a point of philosophical contention during the 17th and 18th Century, the so-called Age of Enlightenment or Age of Reason.
René Descartes took the rather mystical view that, although a material body has the property of spatial extension, it has no inherent capacity for temporal endurance. Rather, God, by his continual action, sustains (or effectively re-creates) the body at each successive instant. Time is therefore a kind of constant process of re-creation by God.
Realism and Anti-Realism
But the two main conflicting schools of thought on the subject during this period are sometimes characterized as realism vs. anti-realism or absolute vs. relational.
The realist viewpoint, championed by Sir Isaac Newton and his fellow Englishmen Isaac Barrow and Samuel Clarke, asserted that time is part of the fundamental structure of the universe, and that it can best be looked on as a dimension in which events occur in sequence (indeed, this view is sometimes referred to as “Newtonian time”). According to Newton, absolute time exists independently of any perceiver, progresses at a consistent pace throughout the universe, is imperceptible, and can only be understood mathematically. Time is therefore an entity in its own right, and we (and all the objects in the universe) are just temporarily “occupying” it. However, as mere humans, Newton cautioned, we are only capable of perceiving what he called relative time, which is a measurement of perceivable objects in motion (like the Moon or Sun), from which we infer the passage of time.
On the other hand, Newton’s great rival Gottfried Leibniz firmly believed that time does NOT refer to any actual existing dimension that “flows” and that events and objects move through in some way. Rather, the anti-realist view of time claims that it is merely a convenient intellectual concept (like space and number) that enables us to sequence and compare events. Thus, time ceases to have any meaning unless there are objects with which it can interact or relate to (for this reason, this view is also known as relational time), so that, in a very real sense, the events taking place actually ARE time. Certainly, time does seem to be a very abstract concept: if we try to think of a moment in time, all we can really do is think of an event that happened at that moment; we cannot point to, or even describe, the moment itself. According to this view, then, time (and space) are merely the product of the way we represent things to ourselves, because we are only capable of knowing objects and events as they appear to us. During the years 1715-6, Leibniz and Clarke carried on an extensive public correspondence, arguing over the (respectively) relational or absolute qualities of time and space.
Another great German philosopher, Immanuel Kant, in his influential 1788 book The Critique of Pure Reason, described space and time as a priori notions that are necessary to allow us to comprehend sense experiences, but not substances or entities in themselves (in his terminology, “phenomenally real” but “noumenally unreal”). He believed that our minds actually structure our perceptions in such a way that space always has a Euclidean geometry, and time always has the linear structure of a mathematical line. Time is therefore merely one element of the systematic framework we use to structure our experience, the one we use to quantitatively compare the interval between, or the duration of, events. So, although empirically real (i.e. not a mere illusion), Kant asserted that time is “transcendentally ideal”.
Although a physicist, Ernst Mach was also relativist in the mold of Leibniz. Writing towards the end of the 19th Century, Mach argued that, even if it was not obvious what time and space were relative to, they were still relative to the “fixed stars” (i.e. the bulk of matter in the universe) if nothing else, and that in an empty universe such concepts would no longer have any meaning.
Specious Present
Historically, the present has usually been considered a momentary, instantaneous, even unmeasurable, time. In the late 19th century, though, E. Robert Kelley (writing under the pseudonym E.R. Clay) started to contest that notion, introducing the concept of the “specious present”. Kelley saw the present as merely the most recent part of the past. The early psychologist and philosopher William James, further developed the idea, describing the specious present as “the short duration of which we are immediately and incessantly sensible”.
James argued that whatever we experience, however fleeting, always has an element of an earlier part and a later part, and anything with an earlier part and a later part cannot possibly be instantaneous in physical time. Another way to look at this is that what an observer sees at any one time is some kind of motion, and motion takes place over an interval, so therefore what the observer sees as present actually occurs over an interval. If a sequence of events occurs over a short enough duration of physical time, we may experience them as simultaneous in perceived or psychological time, but they nevertheless still take place over a short period of physical time.
It seems, then, that, at least to some extent, we perceive events as present that are actually past. In fact, given the fact that the speed of both sound and light are finite, and given that some additional time is needed for the transmission of information from our sense receptors to our brain, it seems that we can only ever perceive what is past. Some proponents of the specious present have gone into even finer arguments over whether or not individual specious presents can overlap each other, and about how they combine to form our stream of consciousness.
Conventionalism
Our everyday use of time relies to a large extent on conventionality. For example, it is a more or less arbitrary convention or convenience that we agree to the practice of re-setting our clock by one hour as we cross a time-zone, that there are twenty-four hours in a day rather than ten, and sixty seconds in a minute rather than twelve, that we measure our dates from the putative date of the birth of a particular religious figure, etc.
The 19th Century French physicist, mathematician and philosopher Henri Poincaré was a great believer that convention plays an important role in physics, and in particular that the geometry of space and time is largely decided by convention, since different geometries may describe a set of objects equally well. This position, referred to as conventionalism, was later taken up by the philosopher of science, Hans Reichenbach, who argued that the whole edifice of relativity (see the section on Relativistic Time) rests on what is effectively a convention, namely that the speed of light is a constant, albeit a convention backed up by the equations of James Clerk Maxwell, and the practical experimentation of the famous Michelson-Morley experiments.
Phenomenology
In the late 19th and early 20th Century, the German philosopher Edmund Husserl, founder of the influential phenomenology movement, began to consider time and our internal consciousness of time. He asserted that we cannot have any perception of the immediate present without some memory of the past and some expectation of the future to give it context.
The French phenomenologist and intuitionist philosopher Henri Bergson formulated a view of time that was neither a real homogeneous medium nor a mental construct, but possesses what he referred to as “duration”. He saw the apparently fluid flow of time as actually composed of a myriad tiny temporal particles, which are pieced together by our consciousness. Noting that as soon as one tries to measure a moment it is gone, and that the passage of time is subjective for different individuals in different circumstances, Bergson concluded that duration (or time) can only be shown indirectly and incompletely, and can only be grasped through a simple intuition of the imagination. Bergson thought of time as something entirely derived from subjective experience, so that babies would not experience time directly (as Kant believed), but rather would have to learn how to experience it. He even went so far as to claim that the modern scientific concept of time as a dimension (see the section on Relativistic Time) actually misrepresents reality.
In his complex but important 1927 book Being and Time, the German phenomenologist and existentialist Martin Heidegger concluded that, we do not exist WITHIN time, but in a very real way we ARE time, and the whole concept of time is inseparable from the human experience. Also, he noted that, because we can allow the past to exist in the present through memory, and even allow a potential future occurrence to exist in the present due to our human propensity to care and be concerned about things, then we are not stuck in simple sequential or linear time, but can step out of it almost at will.