Mach’s Principle

The inertia of any system is the result of the interaction of that system with the rest of the Universe. In other words, every particle in the universe ultimately has an effect on every other particle."

Thus, when the elementary Physics Textbooks say "consider the interaction of two isolated particles" they are living in a different Universe than the rest of us. The "free body diagram" is a useful calculational trick that works sometimes, but denies the basic nature of our universe.