McTaggart distinguished two conceptions of time: the A-series, according to whichevents are either past, present or future; and the B-series, according to which events aremerely earlier or later than other events. Elsewhere, I have argued that these two views,ostensibly about the nature of time, need to be reinterpreted as two views about thenature of the universe. According to the so-called A-theory, the universe is three-dimensional, with a past and future; according to the B-theory, the universe is four-dimensional. Given special relativity (SR), we are obliged, it seems, to accept (a mod-ified version of) the B-series, four-dimensional view, and reject the A-series, three-dimensional view, because SR denies that there is a privileged, instantaneous cosmic"now" which seems to be required by the A-theory. Whether this is correct or not, it isimportant to remember that the fundamental problem, here, is not "What does SRimply?", but rather "What is the best guess about the ultimate nature of the universe inthe light of current theoretical knowledge in physics?". In order to know how to answerthis question, we need to have some inkling as to how the correct theory of quantumgravity incorporates quantum theory, probability and time. This is, at present, anentirely open question. String theory or M-theory, seems to evade the issue, and otherapproaches to quantum gravity seem equally evasive. However, if probabilism is afundamental feature of ultimate physical reality, then it may well be that the A-theoryor rather a closely related doctrine I call "objectism", is built into the ultimate con-stitution of things.
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