This is the heart of the text. Ohanian derives the Einstein Equation ($G_\mu\nu = 8\pi G T_\mu\nu$) using a mix of historical reasoning (Hilbert’s action principle) and physical necessity. He explicitly shows how the Newtonian potential emerges from the weak-field limit, ensuring the student never forgets that GR must reduce to Newton’s law where possible.

"Okay," Ben said. "You win. Not because you were faster. But because you found a book with a pulse."