A-Theory vs. B-Theory of Time

1. McTaggart's Distinction: The A-series and the B-series

The modern debate begins with the Scottish idealist philosopher J. M. E. McTaggart, who, in his 1908 paper "The Unreality of Time," introduced a crucial distinction between two ways of ordering events.

• The A-series orders events according to whether they are in the past, present, or future. These are temporary, monadic properties that an event has. An event that is now future will become present, and then past. The "Battle of Hastings" is in the past; "your reading of this sentence" is in the present; "the first human mission to Mars" is in the future.
• The B-series orders events according to the permanent relations of earlier than, later than, and simultaneous with. The "Battle of Hastings" is earlier than your reading of this sentence, which is earlier than the first human mission to Mars. These relations never change.

To get a clear grasp of this foundational distinction, let's watch a short video that explains it with concrete examples.

As the video explains, the A-series describes a dynamic, changing reality where the property of "presentness" moves from one event to the next. The B-series, in contrast, describes a static, unchanging set of relations between events. Your physics background provides a powerful lens here: the B-series is analogous to the set of fixed coordinate relations between events in a Minkowski spacetime diagram, while the A-series seems to describe something more—an observer's particular journey through that spacetime, with a unique "now" at every point.

2. The A-Theory vs. The B-Theory: Dynamic vs. Static Time

This distinction between the A-series and B-series gives rise to two major, opposing families of theories about the metaphysics of time.

The A-Theory: The Dynamic View

The A-theory of time maintains that the A-series is fundamental. A-theorists believe that the distinction between past, present, and future is an objective, mind-independent feature of reality. Time genuinely passes, or flows. The present moment, the "now," is metaphysically special or privileged. This view corresponds closely with our common-sense experience of time as a moving river that carries us from the past into the future.

A-theorists argue that only their view can make sense of genuine change. For something to change, it must have a property at one time and lack it at another. For the A-theorist, this is explained by the movement of the "now." A banana changes from green to yellow because at one time "The banana is green" is true, and at a later time "The banana is yellow" is true. These tensed facts are fundamental.

The B-Theory: The Static View

The B-theory of time, conversely, holds that the B-series is fundamental. B-theorists believe that the distinctions between past, present, and future are not objective features of the world but are merely subjective or perspectival, like the distinction between "here" and "there." On this view, all moments of time are equally real. There is no objective passage of time, only a static, four-dimensional block of events ordered by "earlier than" and "later than." This is often called the "block universe" theory.

For the B-theorist, what we call "change" is simply variation across the temporal dimension. The banana isn't "first green, then yellow" in some dynamic sense. Rather, the four-dimensional spacetime "worm" that constitutes the banana's history has a temporal part that is green and a later temporal part that is yellow. All facts can be expressed as tenseless propositions, such as "The banana is green at time $t_1$" and "The banana is yellow at time $t_2$".

The following reading from the Stanford Encyclopedia of Philosophy provides a more detailed analysis, including the different kinds of evidence each side marshals. As you read, notice the strong connection between the B-theory and the picture of spacetime in modern physics.

3. Ontological Consequences: What Exists?

The A-theory/B-theory debate is not merely about conceptual descriptions; it has profound consequences for ontology—the study of what exists. The central question becomes: do past and future events exist?

This leads to several specific views, which can be mapped onto the A/B distinction.

Let's examine the main positions, which are neatly visualized in the diagrams and explained in the next video.

To consolidate these ideas, here is a summary and another helpful visual:

• Presentism (A-theory): Only present objects and events exist. Socrates does not exist; future space colonies do not exist. This is ontologically sparse but faces challenges in explaining truths about the past (the "truth-maker" problem).
• Growing Block Theory (A-theory): The past and present exist, but the future does not. The block of reality "grows" as time passes. This captures the intuition that the past is fixed while the future is open.
• Eternalism (B-theory): All events—past, present, and future—are equally real and exist. Socrates and future space colonies are just as real as you are, just located at different temporal coordinates. This is the "block universe" view.
• Moving Spotlight Theory (A-theory): This view accepts the eternalist's block universe but adds that one slice of this block is objectively and uniquely "present." This "spotlight" of presentness moves along the block.

4. A Deeper Look: Is There a Real Difference?

The most subtle and interesting philosophical territory lies in the dispute between the Moving Spotlight theory (an eternalist A-theory) and standard Eternalism (a B-theory). Both agree that all events in the block universe exist. So, what is the actual metaphysical disagreement?

The B-theorist can account for our tensed talk (e.g., "It is now raining") by treating "now" as an indexical, just like "here" or "I". The truth of the statement is relative to the context of utterance.

The A-theorist insists there is more to it. They claim "presentness" is a real, fundamental, non-relational property that an event or time has. The disagreement is about whether there is an objective, privileged present over and above the indexical function of the word "now."

The following reading delves into this very sophisticated problem. It asks what, exactly, an A-theorist who is also an eternalist (like the Moving Spotlight theorist) must believe that a sophisticated B-theorist does not.

This deepens the debate from a simple dynamic vs. static picture to a subtle question about the fundamental furniture of the world. Does reality include a "moving now," or is the block universe all there is, with our sense of passage being a feature of our consciousness journeying through it?

Conclusion

You have now explored the primary divide in the metaphysics of time. This is not just an abstract philosophical game; your stance on this issue has ripple effects, influencing views on causation, change, persistence, and, as we saw in our last lesson, free will.

• Key Takeaways:
  • The debate is rooted in McTaggart's distinction between the A-series (past, present, future) and the B-series (earlier than, later than).
  • The A-theory holds that the A-series is fundamental, time is dynamic, and the present is metaphysically privileged. This view aligns with our everyday experience.
  • The B-theory holds that the B-series is fundamental, time is static, and all moments are equally real. This "block universe" view is highly compatible with the picture of spacetime in modern physics.
  • These theories lead to different ontologies: Presentism and the Growing Block view are A-theories, while Eternalism is the standard B-theory.
  • The Moving Spotlight theory is a hybrid, positing an A-theoretic "moving now" within a B-theoretic eternalist block.

Preview of the Next Lesson:
If we accept a "block universe" ontology (as both Eternalist B-theorists and Moving Spotlight A-theorists do), a new puzzle arises: how do objects persist through time? Are you, for example, a three-dimensional object that is wholly present at every moment you exist? Or are you a four-dimensional object—a "spacetime worm"—that has different temporal parts at different times? This is the problem of temporary intrinsics, and in our next lesson, we will analyze the competing solutions of endurantism and perdurantism.