An Outline of Esoteric Science by Rudolf Steiner; Translated by Catherine E. Creeger
"Esoteric science is the science of what takes place esoterically, in the sense that it is perceived not outside in nature but where one’s soul turns when it directs its inner being toward the spirit. Esoteric science is the opposite and counterpart of natural science." ―Rudolf Steiner
This is essential reading for all serious students of esoteric spirituality.