 Conceptualism (also called psychologism and mentalism, depending on the sorts of objects under discussion): This is the view that there do exist numbers — or properties, or propositions, or whatever — but that they do not exist independently of us; instead, they are mental objects; in particular, the claim is usually that they are something like ideas in our heads. As we will see below, this view has serious problems and not very many people endorse it. Nonetheless, it has had periods of popularity in the history of philosophy. It is very often thought that Locke held a conceptualistic view of universals, and prior to the twentieth century, this was the standard view of concepts and propositions. In the philosophy of mathematics, psychologistic views were popular in the late nineteenth century (the most notable proponent being the early Husserl (1891)) and even in the first part of the twentieth century with the advent of psychologistic intuitionism (Brouwer 1912 and 1948, and Heyting 1956). Finally, Noam Chomsky (1965) has endorsed a mentalistic view of sentences and other linguistic objects, and he has been followed here by others, most notably, Fodor (1975, 1987).