by Carl Jung: From Chapter 9 of Modern Man in Search of a Soul. It was universally believed in the Middle Ages as well as in the Græco-Roman world that the soul is a substance. 
Indeed, mankind as a whole has held this belief from its earliest beginnings, and it was left for the second half of the nineteenth century to develop a “psychology without the soul”.
Under the influence of scientific materialism, everything that could not be seen with the eyes or touched with the hands was held in doubt; such things were even laughed at because of their supposed affinity with metaphysics. Nothing was considered “scientific” or admitted to be true unless it could be perceived by the senses or traced back to physical causes. This radical change of view did not begin with philosophical materialism, for the way was being prepared long before. When the spiritual catastrophe of the Reformation put an end to the Gothic Age with its impetuous yearning for the heights, its geographical confinement, and its restricted view of the world, the vertical outlook of the European mind was forthwith intersected by the horizontal outlook of modern times. Consciousness ceased to grow upward, and grew instead in breadth of view, as well as in knowledge of the terrestrial globe. This was the period of the great voyages, and of the widening of man’s ideas of the world by empirical discoveries. Belief in the substantiality of the spirit yielded more and more to the obtrusive conviction that material things alone have substance, till at last, after nearly four hundred years, the leading European thinkers and investigators came to regard the mind as wholly dependent on matter and material causation.
We are certainly not justified in saying that philosophy or natural science has brought about this complete volte-face. There were always a fair number of intelligent philosophers and scientists who had enough insight and depth of thought to accept this irrational reversal of standpoint only under protest; a few even resisted it, but they had no following and were powerless against the popular attitude of unreasoned, not to say emotional, surrender to the all-importance of the physical world. Let no one suppose that so radical a change in man’s outlook could be brought about by reasoning and reflection, for no chain of reasoning can prove or disprove the existence of either mind or matter. Both these concepts, as every intelligent man today may ascertain for himself, are mere symbols that stand for something unknown and unexplored, and this something is postulated or denied according to man’s mood and disposition or as the spirit of the age dictates. There is nothing to prevent the speculative intellect from treating the psyche, on the one hand, as a complicated biochemical phenomenon, and at bottom a mere play of electrons, or, on the other, from regarding the unpredictable behaviour of electrons as the sign of mental life even in them.
