 Marsilius argued that universal concepts such as ‘humanity’ do not refer to real universals outside the human mind. Accordingly, there is no universal essence in singular individuals. Individuals of one genus or species do resemble each other, however, and this resemblance is the foundation of universal concepts in the human mind. The generation of universal concepts is a natural process, which Marsilius described as follows: suppose individual A of species S evokes concept X in the human mind. This concept is similar to concept Y which has been evoked by B of the same species S. By abstracting from all the differences between X and Y, the human mind is able to produce another concept, Z, which stands for both A and B. Universality is then taken to be a quality of concept Z, the product of the epistemological operation of abstraction on concepts X and Y by the human  mind.[5]