New Essays on Human Understanding

Leibniz's chapter-by-chapter reply to Locke's Essay Concerning Human Understanding, written in French as a dialogue between Philalethes, who speaks for Locke, and Theophilus, who speaks for Leibniz. He set it aside after Locke died in 1704, and it appeared only in 1765, almost fifty years after his own death. The preface states the main disagreements. Against the mind as a blank slate, Leibniz compares it to a block of marble whose veins already mark out the figure of Hercules: ideas and truths are innate as inclinations and dispositions that experience brings out. Necessary truths, such as those of mathematics, cannot be proved by any number of examples. He adds that nothing is in the intellect that was not first in the senses, except the intellect itself. Leibniz also argues for small perceptions too faint to notice, like the sound of each single wave within the roar of the sea, and responds to Locke on substance, freedom, personal identity, language and probability.

It is the great early modern debate between rationalism and empiricism staged as a conversation, and its idea of unconscious small perceptions anticipated later psychology.