I'm pretty sure he meant different by complexity, as the rest of the quote would imply, not by definition. Regardless, I would disagree that they differ so much by complexity, anyway. I'm sure if you were to look into the code of the most advanced physics engines in games... you'd find that the vector mathmatics there was as complicated as any practical math problem. The place where calculus gets fuzzy that practical gaming applications wouldn't really go is where they consider an undefined number of dimensions. I'm not a game programmer, anyway, so I couldn't really comment farther than that.