Time has always posed an intellectual problem. It cannot be observed directly, but is presumed from observed effects (change, motion, etc.). But unlike other inferred phenomena (such as the HIV virus or subatomic particles), time is the basis of our experience of existence and of the way we think. Without time, the most that we can say of something is that it is, yet we haven't discovered anything that simply is (which, if found, we would only be able to identify and describe as not changing over time). As the Buddhists remind us, "all that is subject to arising is subject to cessation." All we can find are processes, change, motion. The closer we look, the less stable existence becomes. And just try to imagine thought and language without any active terms, progressions, causes, or implications. Kierkegaard (who was perhaps overly fond of drawing distinctions) used these presuppositions to separate the temporal (as a perpetual nothing) from the eternal:

If time is correctly defined as an infinite succession, it most likely is also defined as the present, the past, and the future. This distinction, however, is incorrect if it is considered to be implicit in time itself, because the distinction appears only through the relation of time to eternity and through the reflection of eternity in time. If in the infinite succession of time a foothold can be found, i.e., a present, which was the dividing point, the division would be quite correct. However, precisely because every moment, as well as the sum of the moments, is a process (a passing by), no moment is a present, and accordingly there is in time neither present, nor past, nor future.

Presumably the temporal would be the realm of our created existence, while the eternal would be the realm of the divine. This is the sort of dichotomy that provides the bases of dualism. But less than sixty years after Kierkegaard reached those conclusions, Max Planck quantized existence. As described by Julian Barbour in The End of Time (wherein he takes those same presuppositions to suggest that time itself is an illusion), there are characteristic units of existence, defined by Planck's constant:

...[Planck] introduced a new constant of nature, the quantum of action, now called Planck's constant, because the same kind of quantity appears in the principle of least action. Until Planck's work, it had been universally assumed that all physical quantities vary continuously. But in the quantum world, action is always 'quantized': any action ever measured has one of the values 0, 1/2h, h, 3/2h, 2h, ... . Here h is Planck's constant. (The fact that half-integer values of h, i.e. 1/2h and 3/2 h, ..., can occur in nature was established long after Planck's original discovery. By then it was too late to take half the original quantity as the basic unit.) The value of h is tiny...

...When Planck made the first quantum discovery, he noted an interesting fact. The speed of light, Newton's gravitational constant, and Planck's constant clearly reflect fundamental properties of the world. From them it is possible to derive the characteristic mass m_Planck, length l_Planck, and time t_Planck, with approximate values

l_Planck = 10^-33 centimetres
t_Planck = 10^-43 seconds
m_Planck = 10^-5 grams

Couldn't we infer that those characteristic units are fundamental, or, in other words, that 10^-43 seconds is the length of the present? That would be the size of now. And if now has a dimension, we have a foothold in the infinite succession of time. If the present is an identifiable (and theoretically measurable) instant, rather than a non-existent barrier between the past and the future, the implications are significant. Physics and the mathematics that best described it wouldn't be abstract and continuous; they would be concrete and discrete. Something definite would be the basis of our existence, and Kierkegaard's attempt to separate the temporal from eternity will have failed.