The theory of innate ideas expresses a belief that humans come into the world with certain cognitive underpinnings as well as a store of ideas which help us to comprehend reality. Leibniz, in a letter to a friend, identifies not only a faculty of knowledge within man but also a disposition toward it, suggesting a more or less typical human mode by which we characteristically interact with the information we can absorb. In this way, notions regarding innate ideas recall certain ancient arguments for inherent intellectual abilities general to mankind. Locke, on the other hand, believes the infants mind to be a tabula rasa, or clean slate, which begins to build increasingly complex inklings and eventually concepts and ideas only after the outside world has impressed itself through the senses.
Locke defends himself first from the argument that, since there seem to be both speculative and practical principles that mankind can agree upon, it must be the case that this universal consent arises from various persons being borne with inherent ideas that are similar enough. Locke counters that there is nothing which would necessarily make something which is universally consented to also innate, and further contests the validity of the argument by raising the objection that there does not seem to be anything that can gain an absolutely universal consent. Therefore, the argument itself must not have sufficient ground to stand on. Lockes next move is to challenge the basic assumption of the theory of innate ideas by pointing out that children and idiots seem to lack any apprehension or thought whatsoever, which would seem to make an absolutely universal consent impossible even theoretically.
However, in defense of innate ideas, one might say that men can come to know such ideas once they come to the use of reason as a tool of discovery and verification. Locke therefore counters that there are an infinite number of meaningless inanities to which people give their assent through reason every day. He believes that if the statement regarding the use of reason is to helpfully apply to their present dispute, it would either have to assert that upon coming into the age of reason one naturally learns of all their congenital principles, or that the use of reason can prove such ideas to be innate. Naturally, Locke finds both possibilities to be false. What Locke is consistently taking issue with is his very stringent interpretation of the argument for innate ideas that relies on the possibility of universal consent. He takes it perhaps more literally than proponents of innate ideas might consider it themselves, but nevertheless does an ample job of demonstrating that, if indeed the argument relies upon absolute universal consent, it cannot seem to make itself stand. There seem to be too many members of mankind that either physically cannot obtain to a knowledge of these innate ideas, or whose mental habits are not conducive to seeking them out with any degree of accuracy or fidelity to truth.