Metaphysics is the branch of Philosophy that deals with the nature of our
world...the study of being and reality. A
central branch of
metaphysics is ontology, the investigation into what categories of things
are in the world and what relations these things bear to one another. This
time, being, and the nature of existence. . Many philosophers over the
years have tried to understand the notion of
time, using mathematics or logic to explain the way our world works.
The nature of causality is a major topic in metaphysics, although it is one we know very little about. Causality seeks to explain how two events are linked--i.e. if one event caused another. The idea of entropy is a major player in causality. The notion that the universe tends towards disorder is essential in explaining why events transpire. It can be assumed that if the universe tends towards disorder, that one event causing less disorder would lead into an event causing more, however with our current knowledge of physics there is no way to quantify the entropy of most events (those events that are not reactions--how does one measure the entropy of war?) Causality remains an aspect of time that has been argued by many, but it seems like no headway will be made until our understanding of physics increases.
Among groups of metaphysicists, there are two general groups, tensed theorists and tense-less theorists. Tensed theorists believe that the passage of time is an objective fact; that time flows from past to present to future and there is no subjectivity to it. Tense-less theorists believe that the only objective view of time is when it is used in the relation of two objects. This division between theories is one of the major issues concerning metaphysics.
Another issue is the idea of causal asymmetry. A long standing idea in metaphysics was the idea of causal asymmetry, which is the idea that events in the future cannot effect those in the past, but those in the past can affect events in the future. This is a simple idea in itself, but attempts to explain causal asymmetry in terms of other methods of understanding time have not proven fruitful. Some metaphysicists believe that causation may actually be symmetric, but that some aspect of our psychological constitution makes it appear asymmetric. The idea that causal asymmetry can be explained partly by our psychological constitution implies that other seemingly objective things can be challenged in the same way.