Rationalism and empiricism are distinct epistemologies. A fundamental difference between rationalists and empiricists concerns how each thinks you arrive at knowledge. For one, the two views differ over whether you possess innate (inborn) ideas, prior to your actual experiences. Rationalists maintain that you have knowledge of reality prior to your experiences. Put another way, you don't need sensations to have knowledge. Empiricists like Locke think differently. According to Locke, your ideas of qualities like redness and sweetness and of entities like triangles begin with sensations of those objects. There is no knowledge prior to sensation.

Like other empiricists, Locke said knowledge is imprinted on the mind by sensations. Locke maintained that these sensations impress themselves upon the mind as if it were a tabula rasa, a blank table or “white paper void of all characters, without any ideas,” as he put it. Where do all the ideas — that is, all the materials in reasoning and knowing — come from? In a word, they come from experience; hence the name “empiricism.”

There are two kinds of experience, external and internal, and two corresponding paths to knowledge: (1) sensations provide you with ideas emanating from external entities and experiences (from objects outside yourself); (2) inner reflection also provides ideas as part of the world within. Your mind receives ideas in the same way that a blank page receives pen marks on it. Sensation or “sense experience,” as Locke refers to it, presents sensible qualities to the mind, such as cold, blueness, and softness. When the mind reflects on these sensations, it receives a second set of ideas pertaining to these sensory operations. These include perception, thinking, doubting, believing, reasoning, knowing, and willing.

No Innate Ideas

As with other philosophers of the seventeenth and eighteenth centuries, Locke was fascinated with questions about the limits of the human mind. In his “Epistle to the Reader” he introduces the subject matter, saying that the job of the philosopher is that of an “under-laborer” who must clear the ground and remove “some of the rubbish in the way of our knowledge.” He thought that rationalism had claimed that the power of reason to know things was greater than it actually was and had actually “meddled in things” that exceed the mind's comprehension.

In his Essay, which comprises four large books, Locke began his argument by pointing out that while innate ideas are universally accepted by humankind, such universal agreement does not prove them to be innate. Even manifest principles of logic such as the principle of identity (“Everything is equal to itself”), the principle of contradiction (“It is impossible for the same thing to be and not to be”), and the principle of the excluded middle (“Something either is or it isn't”) are not even known to “children and to idiots,” Locke says. So how could we have known such principles at birth?

Further, when someone maintains that he knows mathematics and logical truths a priori — that is by reason and not by experience — this hardly proves that such notions are innate. Even moral principles — such as the Golden Rule or particular moral propositions like “Stealing is wrong” — are not known innately, for they require proof.