It is natural that our senses should have developed faculties for perceiving some of these intrinsic distinctions of the possible states of the world around us. I prefer to think of matter and energy, not as agents causing the degrees of curvature of the world, but as parts of our perceptions of the existence of the curvature.

Space, Time and Gravitation: An Outline of the General Relativity Theory - Chapter V (p. 92), At The University Press. 1921