Time is not the 4th dimension. Well... depends on perspective. Why do people say time is the 4th dimension? I know you can say it's a dimension and there are already 3. But why not say mass is a dimension? Each point in space has a certain mass. What's the difference? Einstein said it and it became convention? That better not be the reason . I think it would be best when describing time with space and need to call it a dimension to say something like, "If you use time as a dimension then..."