 The story so far is that Newton took his first law from Galileo, who said that if you don't apply a force to something, it doesn't change its velocity. And then he developed his second law, which makes the first law really just a special case of his second law, which tells you exactly how things accelerate when you apply forces to them. So the mass times acceleration is equal to the sum of the forces acting on something. So if you want to know how something moves, all you need to know is all the forces. And Newton's second law is correct provided you don't go too fast so that relativity becomes important, or you don't look at things that are too small so that quantum mechanics becomes important. So it's a very, very powerful tool for calculating things. But you do need to know all the forces that apply to things. And one of the ways to help that is to look at Newton's third law, which tells us an important rule that forces have to obey. Fundamentally, all forces come from interactions. So a force arises where you have something, here I have something A, and something else. B, and they have some kind of interaction. Now we only know of a few fundamental kinds of interactions. We know about gravity, we know about electromagnetism, we know about the nuclear strong force and the nuclear weak force. But all forces, no matter where they come from, all follow Newton's third law. Newton's third law says that all forces come in pairs. So the force on particle A due to particle B has to be paired with an equal and opposite force on particle B due to particle A. So in other words, you've got one force going one way, one force going the other way. They're exactly the same magnitude, so if you add them up you'll get zero. And that's Newton's third law.