I was talking to my friend (who's a physics major) about the difference between quantum and classic mechanics. Planck's constant came up somehow and my friend said that you could use it to determine with a system can be described by classical or quantum mechanics. I thought he was crazy since it's just a constant, how can it determine anything? That's like saying the speed of light can determine how fast a system is moving... it doesn't make sense. Anyone have any comments? Is he right or making everything up?