Ideas that are inborn and not the product of experience. The controversy over their existence formed a major element in Locke's rejection of the philosophy of Descartes, and Locke was in turn attacked over the issue by Leibniz. The question was whether the mind is equipped with highly general concepts—God, freedom, immortality, substance— that can be known a priori to be applicable to the world, and that could therefore afford us clear and distinct knowledge unlike any that can be certified by experience. It thus formed a pivotal issue between rationalists and their empiricist opponents. The connection between innateness as such and any right to think of these ideas as yielding a distinct kind of knowledge was queried by Hume, but the need for a priori categories of the understanding through which experience is organized was again defended by Kant. In the 20th century the view that children carry with them an innate universal grammar, enabling them to pull off the remarkable feat of learning a first language, has been defended by Chomsky.