The theory of innate ideas was a common one during Locke's time; it is basically the theory that certain pieces of knowledge exist in us from (or before) birth, and are not gained from experience of the outside world. Descartes relied heavily on the theory of innate ideas to justify his system, by using them to argue for the existence of God. He claimed that we all have an innate idea of the existence of God and His basic nature. Other philosophers often used them to argue for their particular system of morality, claiming that we had innate moral rules implanted in us at the moment of creation by God. The theory of innate ideas can be loosely traced back to Plato, who claimed we first gain fundamental knowledge of things (knowledge of 'the forms') before we are born, and gradually recall or relearn them during our lifetime. Both Plato and Descartes seem to be influenced by the phenomena that Descartes calls 'clear and distinct' perceptions. These are facts which as soon as they are explained to us seem so self-evident that we cannot but accept them immediately and declare that they could not be other than true. A good example is that a thing cannot exist and not exist at the same time. Typically proponents of the theory held that it seemed more like we were relearning or remembering these ideas rather than hearing them for the first time.

Locke effectively attacks this theory in the first section of his 'An Essay Concerning Human Understanding'. He does this because the project of his Essay is to prove that knowledge comes from external sense perception, not from pure reason alone (as Descartes had argued), and most especially not from ideas which we gain before having any sense experience. He first attacks the idea of universal assent. Proponents of innate ideas claimed that the fact that certain ideas were held to be true by everyone, proved that they were innate. Locke points out that even if an idea were universally accepted, this would not prove its innateness, unless there was no other way for that idea to discovered and accepted. For example, that a triangle has three sides needs not be innate to be universally accepted. Anyway he argued that there are no universally accepted ideas. Children have no notion of the truth that 'a thing cannot be and not be at the same time', so it obviously cannot be innate in them. Moral rules are even more straightforward; even leaving aside the issue of those seemingly unburdened by a conscience, who could seriously argue that different cultures often have totally different rules, and what is for one pedestrian, is for another completely unacceptable?

Locke also points out that if there are innate ideas, why do we not all know them? There should be no question as to how many there are, or what they are, or most especially if they even exist, since being innate we should all know them automatically. That we do not, Locke argues, is a commonsense objection to the whole theory.

Objections

The supporters of innate ideas would argue that innate ideas are not known to us immediately, but require the use of reason to discover them. In our modern terminology this would make them something like knowledge held in the subconscious. Locke thought little of this retort, for he argued that to say that innate ideas need to be discovered, means that there is no difference between innate ideas and any other ideas, so where is the proof that they are different? To argue that ideas are innate merely because everyone to whom they are explained immediately grasps and agrees to them, shows nothing more than that they are simple and obviously correct. Locke points out that supporters of innate ideas do not believe that truths like '2 and 3 equal 5' are innate, yet those are just as quickly and universally accepted as 'a thing cannot be and not be'.

A reply from proponents for innate ideas to these objections was that modern society and education corrupt the innate truths we initially know. Descartes in particular argued for this view. Locke replied that under this objection children and more primitive cultures (Locke often liked to use Native Americans for such examples) ought to have a clearer and better conception of these innate ideas than we do, for their minds cannot have been corrupted by society. Yet just the opposite seems to be the case. In addition Locke wondered what kind of God would think information important enough to innately implant in us, and yet then leave it open to such radical corruption - commonsense tells us this is absurd.

In large part because of Locke's attacks on innate ideas, the theory soon fell out of favor, and today virtually no philosophers seriously hold that there are innate ideas.