Let's see if I think correctly first: I think a vector is a group of numbers independent of each other. What we say 3D vector means "it takes three numbers to specify a position and these numbers are not (explicitly) dependent on each other. The so called 'direction' of a vector is a visualisation that reflects this property." If the above is correct, and since time is also independent of spacial coordinates, then why can't time be a vector? eg: in the definition of velocity as dx/dt, x is a vector while time is a scalar. Why so?