"By what incomprehensible mechanism are our organs held in subjection to sentiment and thought? How is it that a single melancholy idea shall disturb the whole course of the blood; and that the blood should in turn communicate irregularities to the human understanding? What is that unknown fluid which certainly exists and which, quicker and more active than light, flies in less than the twinkling of an eye into all the channels of life,— produces sensations, memory, joy or grief, reason or frenzy,— recalls with horror what we would choose to forget; and renders a thinking animal, either a subject of admiration, or an object of pity and compassion?"
Psychology
Philosophy