Since antiquity two images of time have been discussed: the line made up of stationary points and the flow of a river. Philosophically speaking, these images correspond to two positions: “being as timeless” and “being as temporal.” The two positions can be found in early Indian thought, for instance, as held in Brahmanism and Buddhism, respectively. The different schools in the Brahmanical tradition have maintained that the ultimate being is timeless (i.e., uncaused, indestructible, beginningless, and endless). Buddhists, on the other hand, have claimed that being is instantaneous and that duration is a fiction since according to their view a thing cannot remain identical at two different instants (Balslev, p. 69 ff.).
In classical Greek thought the tension between the dynamic and the static view of time has been expressed, for example, by the Aristotelian idea of time as the number of motion with respect to earlier and later—an idea that comprises both pictures. On the one hand time is linked to motion (i.e., changes in the world), and on the other hand time can be conceived as a stationary order of events represented by numbers. This discussion is also reflected in Isaac Newton’s (1642–1727) ideas of time, according to which absolute time “flows equably without relation to anything external” (Principia, 1687). The basic set of concepts for the dynamic understanding of time are past, present, and future.
After J. M. E. McTaggart’s analysis of time in “The Unreality of Time” (1908), these concepts (i.e., the tenses) are called the A-concepts. They are well suited for describing the flow of time, since the present time will become past (i.e., flow into past). The basic set of concepts for the stationary understanding of time are before, simultaneously, and after. Following McTaggart, these are called the Bconcepts, and they seem especially apt for describing the permanent and temporal order of events.
Philosophers discuss intensively which of the two conceptions is the more fundamental for the philosophical description of time. The situation can be characterized as a debate between two Kuhnian paradigms: the ideas embodied by the well-established B-theory, which were for centuries predominant in philosophical and scientific theories of time, and the rising A-theory, which in the 1950s received a fresh impetus due to the advent of the tense logic formulated by Arthur N. Prior (1914– 1969). Still, many researchers do not want to embrace the A-conception. According to A-theorists, the tenses are real, whereas B-theorists consider tenses to be secondary and unreal. According to the A-theory the “Now” is real and objective, whereas the B-theories consider the “Now” to be purely subjective.
Following the ideas of Aquinas, some argue that time from God’s perspective should be understood in terms of B-concepts because time is given to God in a timeless way. But it should be mentioned that Aquinas also maintained that divine knowledge can be transformed into the temporal dimension by means of prophecies. It seems that Aquinas was suggesting a distinction between time as it is for temporal beings such as humans and time as it is for God, who is eternal.