It’s relaxing to think that knowledge has no sell-by date: that once you have learned something, it stays fresh forever and you never have to learn it again. In the sciences like math and physics, and in the arts, that is often true. In those subjects, what we all learned at school (2 + 2 = 4) is probably still good. But in the social sciences, even the most basic knowledge goes off very quickly. As with milk or vegetables, you have to keep getting it fresh. Because everything changes.
