In this chapter we discuss a shift in Chomsky’s thinking about the extent to which the acquisition of language is based on a language-domain-specific innate system. The initial idea was that children develop their mental grammar based on two factors: a “richly articulated” innate system, called Universal Grammar (UG), and the language input. Chomsky later decided that the innate language faculty can be reduced to a single operation, “recursive Merge.” This made it necessary to acknowledge a third category of factors that plays a sizeable role in the emergence of mental grammars. These third factors cover a mixed bag, including “general learning systems” (those that empiricists would always have emphasized) and another kind of factor, which Chomsky finds more interesting: “natural laws of form” that are grounded in the laws of physics and perhaps ultimately in mathematical principles. We will discuss this notion of third factors, and I will show that attempts to explain the structure of human mental faculties in terms of principles that determine much, if not everything, in the natural world (both mind-internal and mind-external) are widespread and have a long tradition.