I'd say closures are one of the fundamental concepts of computer science. They're described in the introduction to chapter 2 of SICP, "Building Abstractions with Data".

SOLID sounds nice but I don't think I've ever seen anyone adhere to it religiously. Easy exercise: take any Java program, and imagine the spec changed the width of integers it needs to support, from 32-bit to some other size. How many files would you need to touch to make this change? Clearly, no class in the program has the Single Responsibility of this. The first letter of SOLID is already violated.