In Galilean transformation,it is said that time is a physical quantity while space is dimension. What do we mean by this i.e. time is a physical quantity while space is dimension ? Isn't space a physical quantity in Galilean transformation? Here, too, we plot graph of position(on y-axis) as a function of time (on x-axis). In Lorentz Transformation, it is said that both space and time are dimension. I don't understand that what difference does it bring to our understanding of the concept "time"when we say time is dimension and when we say time is not dimension? How does Lorentz Transformation lead us to the conclusion that time is dimension and to the concept of space - time? Here, too, we plot graph of position(on x-axis) and time (on y-axis). Do the two plots differ? If yes, in what way? How is space- time different from space and time? Does one need to understand these concepts well for solving numerical problems?