The problem of time is one of the most difficult and most keenly debated in the field of natural philosophy. To arrive at a satisfactory orientation in regard to this discussion, it is important to distinguish two questions:
• What are the notes, or elements, contained in our subjective representation of time?
• To what external reality does this representation correspond?
(1) As to the first question, philosophers and scientists in general agree in this: that the notion, or concept, of time contains three distinct ideas fused into one indivisible whole.
• First there is the idea of succession. Every mind distinguishes in time the past, the present, and the future, that is parts which essentially exclude simultaneity and can be realized only one after the other.
• Again, time implies continuity. Speaking of events here below, in our own life, we cannot conceive the possibility of an interval of duration, however short, in which we should cease to grow older, or in which moment should cease to follow moment. The march of time knows neither pause nor interruption.
• Lastly, a continuous succession cannot be a continuous succession of nothing. Therefore the concept of time represents to us a reality the parts of which succeed each other in a continuous manner. It matters little here whether this reality is purely ideal, or is realized outside of us, for we are dealing only with the concept of time.
Such are the three essential elements of the subjective representation. From these considerations it appears that the question of time belongs to the domain of cosmology. By reason of its character as continuous, successive, divisible, and measureable, time belongs to the category of quantity, which is a general attribute of bodies, and cosmology has for its object the essence and general attributes of matter.
(2) The second question, relating to the objectivity of the concept of time, is one upon which philosophers, as well as scientists, are divided: no fewer than fifteen different opinions may be enumerated; these, however, may be grouped in three classes. One class embraces the subjectivist opinions, of which Kant is the chief representative; these regard time as completely a creation of the knowing subject. To Kant and his followers time is an a priori form, a natural disposition by virtue of which the inner sense clothes the acts of the external senses, and consequently the phenomena which these acts represent, with the distinctive characteristics of time. Through this form internal and external phenomena are apprehended by us as simultaneous or successive, anterior or posterior, to one another, and are submitted to necessary and universal time-judgements. To this class, also, belong a group of opinions which, without being so thoroughly subjective, attribute to time only a conceptual existence. To Leibniz and others time is "the order of successions", or a relation between things that follow one another; but if these things are real, the mind perceives them under the form of instants between which it establishes a relation that is purely mental. According to Balmes, time is a relation between being and non-being; subjective time is the perception of this relation; objective time is the relation itself in things. Though the two ideas of being and non-being are found in every succession, the relation between these two ideas cannot represent to us real continuousness, and therefore it remains in the ideal order. Locke considers time as a part of infinite duration, expressed by periodic measures such as the revolution of the earth around the sun. According to Spencer, a particular time is the relation between two in the series of states of consciousness. The abstract notion of a relation of aggregated positions between the states of consciousness constitutes the notion of time in general. To this relation Spencer attaches an essentially relative character, and attributes relative objectivity to psychological time alone. For Bergson homogeneous time is neither a property of things nor an essential condition of our cognitive faculty; it is an abstract schema of succession in general, a pure fiction, which nevertheless makes it possible for us to act upon matter. But besides this homogeneous time, Bergson recognizes a real duration, or rather, a multiplicity of durations of unequal elasticities which belong to the acts of our consciousness as well as to our external things. The systems of Descartes and of Baumann must also be classified as idealistic.
In opposition to this class of opinions which represent the existence of time as purely conceptual, a second class represent it as something which has complete reality outside of our minds. These opinions may fairly be described as ultra-realist. Certain philosophers, notably Gassendi and the ancient Greek Materialists, regard time as a being sui generis, independent of all created things and capable of surviving the destruction of them all. Infinite in its extension, it is the receptacle in which all the events of this world are enclosed. Always identical with itself, it permeates all things, regulating their course and preserving in the uninterrupted flow of its parts an absolutely regular mode of succession. Other philosophers, e.g. Clarke and Newton, identify time with the eternity of God or regard it as an immediate and necessary result of God's existence, so that, even were there no created beings, the continuation of the Divine existence would involve as its consequence, duration, or time. These ultra-realist philosophers substantialize time; others again make it a complete being, but of the accidental order. For de San time is an accident sui generis, distinct from all ordinary accidents; it is constituted as the local movement of parts which succeed each other in a continuous manner, but with perfect uniformity; by this accident, which is always inherent in substance, being and the accidents of being continue their existence enveloped in a succession which is everywhere and always uniform. Lastly, according to Dr. Hallez, the substantial existence of beings itself increases intrinsically without cessation, and this regular and continuous increase is by no means occasional or transitory, but always remains a veritable acquisition to the being which is its subject. Of this quantitative increment time is the representation. To sum up, all systems of this second class have as their distinctive characteristic the assertion of an external concrete reality--whether substantial or accidental--which adequately corresponds to the abstract concept of time, so that our representation of time is only a copy of that reality.
Between these two extreme classes of opinions is the system proposed by the majority of the Scholastics, ancient and modern. For them the concept of time is partly subjective, partly objective. It becomes concrete in continuous, notably in local, movement; but movement becomes time only with the intervention of our intelligence. Time is defined as the measure of movement according to an order of anteriority and posteriority (numerous motus secundum prius et posterius). Once local movement is divided into parts by thought, all the elements of the concept of time are found in it. Motion, being objectively distinct from rest, is something real; it is endowed with true continuity; nevertheless, in so far as it is divided by the intelligence, it contains successive parts actually distinct among themselves--some anterior, some posterior--between which we place a fleeting present. In the elaboration of the idea of time, therefore, movement furnishes the intelligence with a successive, continuous reality which is to be the real object of the concept, while the intelligence conceives it in that which it has in common with all movement--that is without its specific and individual notes--and makes it, formally, time, by dividing the continuity of the movement, making actual that distinction of parts which the movement possesses only potentially. In fact, say the Scholastics, we never perceive time apart from movement, and all our measures of our temporal duration are borrowed from local movement, particularly the apparent movement of the heavens.
Whatever be its objectivity, time possesses three inalienable properties. First, it is irreversible; the linking of its parts, or the order of their succession, cannot be changed; past time does not come back. According to Kant, the reason of this property is found in the application to time of the principle of causality. As the parts of time, he says, are to each other in the relation of cause to effect, and as the cause is essentially antecedent to its effect, it is impossible to reverse this relation. According to the Scholastics, this immutability is based upon the very nature of concrete movement, of which one part is essentially anterior to another. Secondly, time is the measure of events in this world. This raises a knotty problem, which has so far not been theoretically solved. Time can be a permanent measure only if it is concretized in a uniform movement. Now, to know the uniformity of a movement, we must know not only the space traversed, but the velocity of the transit, that is the time. Here there is unquestionably a vicious circle. Lastly, for those who concretize time in movement, a much debated question is, whether time or movement can be infinite, that is without beginning. St. Thomas and some of the Scholastics see no absolute impossibility in this, but many modern thinkers take a different view.