The Pursuit of Happiness (A Book of Studies and Strowings)

Pleasure and pain are both ultimate and undefinable experiences of the mind. We cannot resolve them further; but we can note certain unfailing relations they bear to the organism, which explain their significance. Pleasure characterizes the normal and unimpeded exercise of physiological functions of all kinds. There are as many elementary pleasures as there are sensations. Pain is present only in the reverse conditions. Modern physiologists have established, therefore, the fundamental law, that pleasure connects itself with vital energy, pain with its opposite; in which they have not gone beyond, even if they have caught up to, the maxim of Spinoza: "Pleasure is an affection whereby the mind passes to a greater perfection; pain is an affection whereby it passes to a lesser perfection."