The essence of matter is gravity; the essence of Spirit is Freedom.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“All these quotations are from the introduction to The Philosophy of History. Spirit, and the course of its development, is the substantial object of the philosophy of history. The nature of Spirit may be understood by contrasting it with its opposite, namely Matter. The essence of matter is gravity; the essence of Spirit is Freedom. Matter is outside itself, whereas Spirit has its centre in itself. “Spirit is self-contained existence.” If this is not clear, the following definition may be found more illuminating: “But what is Spirit?” source