Time is a scale and a scalar in physical definition. With time, we can determine the sequence of events according to the past-present-future sequence (time point), and we can also measure the duration of events and the interval between events (time period). Time is the fourth dimension besides the three dimensions of space. Time is a thinking concept in the human brain, which is used to describe the degree of change of things. It should not be confused with the dimension of space. Four-dimensional space defines space and has nothing to do with time.