William Clifford, a British physicist and mathematician, inspired by Riemann's notion of a manifold, a generalisation of non-Euclidean geometry, published in 1876 a small note in the Proceedings of the Cambridge Philosophical Society titled The Spacetime Theory of Matter. Here he wrote a full fifty years before Einstein formulated General Relativity, the following:
- That small portions of space are in fact of a nature analogous to little hills on a surface which is in the average is flat; namely, that the ordinary laws of geometry are not valid on them.
- That this property of being curved or distorted is being continuously being passed from one portion of space to another after the manner of a wave.
- That this variation of the curvature of space is what really happens in the phenomena which we call the motion of matter, whether ponderable or etherial.
- That in the physical world nothing else takes place but this variation, subject (possibly) to the law of continuity.
The qualitative parallel with Einsteins General Relativity is obvious. It's an open question whether Einstein was aware of Cliffords hypothesis. It wouldn't be any surprise if he was and nor, if he was, does it diminish his genius.
More generally, Clifford was answering a question of Newton as to how the force of gravity was transmitted. After all, Newton formulated gravity as 'an action at a distance' - meaning he couldn't formulate a way for the influence to travel tgrough empty space. This principle is called locality. Carlo Rovelli in his book, Quantum Gravity, considered it one of the supreme principles of physics. Einstein certainly felt so, it was the content of his dispute with Bohr on non-locality in QM. He simply couldn't envisage a non-local theory.
However, this principle is much older than that. It goes back to Aristotle in two ways: first he said forces are transmitted by touch and second, he said there was no such thing as a void (he gave five arguments for its non-existence).
So basically, physicists since Newton were looking for a local completion of Newtonian gravity. It was Faradays notion of a field that provided the key via Maxwells equations (Maxwell explicitly credited Faraday for the notion) as well as the tensor calculus of Riemann and Levi-Civita.