An important feature marking Peirce's thought is its continual war against psychologism. This term may in his case be defined as the practice of positing vague, irreducible categories of mind to answer questions that in fact require precise categories of logic. In philosophy proper we can see Peirce's anti-psychologism in his backing of realism against nominalism and in his opposition to Cartesian rationalism. With respect to his semiotics the most important instance is his idea of the interpretant, which is not conceived primarily within a psychological context, i.e. the individual mind interpreting a sign, but rather a context of external relation between one sign and another. The relation is one of mutual conversion: one sign acts as the interpretant of another sign and vice-versa.