Time is a central and classical theme in philosophy. It can be taken up from several angles, including epistemology (how do we know of the existence of a past or a future?), phenomenology (the time lived and perceived as opposed to external time), philosophy of language (the relevance of tenses in communication), philosophy of mind (e.g. time and memory), ethics (e.g., human life ought to be lived with the horizon of our death in view), social philosophy (the importance of recurring rituals in the structuring of a society, e.g. bells, New Year's, 4th of July …) In this article we shall look at time from a metaphysical point of view.
Metaphysics of Time: Three Views
The key question with respect to the metaphysics of time concerns the reality of the past, the present, and the future. Three main views can be traced in this respect.
(i) Eternalism. According to eternalists, past, present, and future all exist. This view is the one that comes closer to represent reality as God would see it: a manifold containing every event, past, present, and future. God's omniscience, indeed, means that God knows everything that has happened, that happens, and that will happen. For an eternalist. the present is like a spotlight, which moves across a fixed reality. Under this perspective, the possibility of genuine free will seems hard to explain, as is the possibility that some natural laws are probabilistic.
(ii) Presentism. According to presentists, there is only the present. Truths regarding the past have to be construed on the basis of what is true of the present moment: if I exist now, it must be because someone gave birth to me; but this truth regarding the past is provable only from the perspective of what we can presently observe. As for the future, it's all to be made. This view has some difficulties coming to grip with relativity theory, in which what is present is relative to the observer - hence there seems to be more than one "present moment" in reality, contra what the presentists claim.
(iii) Open future. According to open future supporters, the past and present are fixed, while the future is open. There are two takes on how to account for the openness of the future. For the Growing Block View (an ancient position, which can be found also in Aristotle), as time passes reality grows. A more recent proposal, instead, claims that the future is branching - hence the name: Branching Time Theory. According to branching time theorists, at each present instant the future holds several possibilities, represented as branches of a tree. Some authors take those branches to be real, some to be mere possibilities: hence, the former variety of branching time theorists are really eternalist, while the latter are genuine open future theorists.
Metaphysics of Time and Knowledge of Things in Time
As always, a metaphysical view is deeply entrenched to an epistemological position (what can be known, really?) and a perspective on language. When it comes to time, there is the question of our epistemic accessibility to the past and the future, which seems to motivate presentism or open future. After all, we can speak of the past only moving from the evidence (memories, traces …) we have, in the present, of a past. And - as Hume neatly pointed out - we can speak of a future only in so far as we believe that future events will resemble past events, as it happened in the past.
On the other hand, an eternalist will see a deep divide between the epistemology and metaphysics of time: the fact that we do not have direct epistemic access to past and future events does not make them less real. This claim, however, will rest unproved. (Time travel, perhaps, could bring more plausibility to it.)
Metaphysics of Time and Speaking of Things in Time
Considerations from philosophy of language are also crucial to our metaphysics of time. There are two ways of speaking of things in time: tensed and tenseless. The latter takes place when we look at things in time as if we were "outside" of it (e.g., when we tell someone the story of our life in those terms: "Here I am celebrating my sixth birthday; here I am getting my high school degree; here I am laying on the beach in Cancun …") Tensed speech occurs when we speak of things in time from the point of view of a specific present. If I say: "I will go to the park this afternoon", I am speaking from the perspective of a present - my present; but we can speak tensedly from the perspective of someone else: "Anne thought that yesterday was Wednesday, but it really was a Tuesday."
Eternalism seems to sit well with tenseless speech: to them, the metaphysics of time is tenseless (that is, the best way to describe reality is tenseless). Presentism sits most naturally with tensed speech. Open future theorists are found of both camps.