The main trick in machine learning
I strongly disagree with the idea that validation sets are central to machine learning. The whole point of machine learning (usually) is to predict things well. Validation sets are merely one technique among many to gauge how well your predictions are doing. Because they are so easy, they are very common. But just because they are common doesn't mean they are central to the field. There are many other techniques out there, like Bayesian model selection (as the author mentions at the end).