One philosophical doctrine – which historically has gone by various names – denies the existence of reason and recognizes only consciousness and external perception. A version of this doctrine, called sensualism, derives everything from sensation. This theory, which was advanced by Democritus and later by Epicurus and the Stoics, explains knowledge as a function of idea-images. Working from the assumption that the only action is that of like producing like, the sensualists hold that the soul, like the body, is material. Nevertheless, the soul remains distinct from the “atoms” that bodies in space throw off from themselves. These atoms, or εἴδδωλλα (images), are like condensed images of the bodies, and as they strike us the images become imprinted on the soul, leaving impressions representing the bodies from which they emanated. These impressions are ideas.

Over the years, the crudeness of this theory was gradually recognized. To improve it, the notion of consciousness was added to external perception, so that knowledge might be derived from experience alone. This doctrine, initially formulated by Locke, is called empiricism. According to the empiricists, the mind prior to experience is like a wax tablet on which nothing has yet been written – a tabula rasa, or blank slate.

More recently, an even stronger version of empiricism has been developed in England. Because it grants an important role to the association of ideas, this version is called associationism.