Materialism believes that human cognition is a reflection of objective things, and they will say that time, as a category of consciousness, is a reflection of objectively existing time. Empiricism, on the other hand, holds that knowledge and theory derived from experience are only consciousness, and we cannot judge whether there are corresponding objective facts, which are questions forever. Materialism may be right, but we do not need to prove whether consciousness is a reflection of objective facts, we only need to prove whether this knowledge is beneficial to mankind. As long as a theory can be self-justified, predictable and simple, it is a theory that is beneficial to human being.

People have put forward many objective explanations for time, but they can not explain objectively what time is. Simultaneous and sequential in time must be defined by consciousness, and objective definition can never explain the nature of time. The definition must define the unknown with the known. Time, as one of the most basic concept, must be reduced to the most basic knowledge of people. The most basic knowledge of people is direct experience.

Kant was right to note the subjectivity of time, but he said, "Time is not an empirical concept that is somehow abstracted from experience." "Time is given innately." (Immanuel Kant. Critique of Pure Reason(In Chinese). Translated by Li Qiuling. Beijing: China Renmin University Press,2005, 2nd Edition, pp.64-65.) It is wrong to deny the subjectivity of time. Time must be defined by direct experience. But direct experience is not entirely self-generated by consciousness, it has an objective source. And its content is independent of man's will, so time has objectivity.

The relationships of "before" and "after" defined by direct experience is the basic time relation. What scientists tell us about the time relationship of the birth and change of the universe is indirect experience, and indirect experience needs to be tested by direct experience.

The simultaneity of indirect experience is the association after logical reasoning. If two things A and B cannot be observed at the same time, but A and C, B and C can be observed at the same time respectively, and the elements in them are respectively simultaneous, then it can be inferred that some elements of A and B are simultaneous. Then C becomes the intermediary for judging simultaneity. The sun and moon are easy to observe and their positions change continuously, so humans first use them as intermediaries to judge simultaneity. Later, the judgment of simultaneity became more and more precise, and humans created a more easily observed intermediary - the clock. For example, if you tell me that you got on the plane at 1:00 yesterday, and I remember that I was eating at 1:00 yesterday, and then I believe that I was eating when you got on the plane, an association is established between these two experiences. The results of these logical reasoning can all be wrong and need to be checked.

Definition: The clock is the intermediary for judging simultaneous and sequential relationships.