Time is what we use a clock to measure. Information about time tells us the durations of events, and when they occur, and which events happen before which others. Nevertheless, despite 2,500 years of investigation into the nature of time, there are many unresolved issues.

Da UnB Agência

Consider this one issue upon which philosophers are deeply divided: What sort of ontological differences are there among the present, the past and the future? There are three competing theories. Presentists argue that necessarily only present objects and present experiences are real, and we conscious beings recognize this in the special vividness of our present experience compared to our dim memories of past experiences and our expectations of future experiences. So, the dinosaurs have slipped out of reality even though our current ideas of them have not. However, according to the growing-past theory, the past and present are both real, but the future is not real because the future is indeterminate or merely potential. Dinosaurs are real, but our future death is not. The third theory is that there are no objective ontological differences among present, past, and future because the differences are merely subjective. They come from us. This third theory is called “eternalism.”

Here is a list of other issues, in no particular order: •What time actually is; •Whether time exists when nothing is changing; •What kinds of time travel are possible; •Why time has an arrow; •How to correctly analyze the metaphor of time’s flow; •Which features of our ordinary sense of the word "time" should be captured by the concept of time in physics; •Whether contingent sentences about the future have truth values now; •When time will end; •Whether there was time before the beginning of our big bang; •Whether the big bang was an expansion of space or of both space and time; Whether tensed facts or tenseless facts are ontologically basic or fundamental; •What the proper formalism or logic is for capturing the special role that time plays in reasoning; •Whether there are points of time; •What neural mechanisms account for our experience of time; •Which aspects of time are conventional; •How time is related to mind; •Whether there is a timeless substratum from which time emerges; •Why our universe has time instead of no time; •Whether the concept of time is objective; •Whether time is an illusion; and •How to settle the dispute between advocates of the A-theory and the B-theory of time.