<p>This paper is about a calculus of ideas referenced in short as <i>idea calculus</i> (<i>IC</i>). The <i>IC</i> has been here formulated in accordance with the view on ideas as mental objects presented in earlier publications of current author. However, this paper comes with improved conceptuality and terminology and should be treated as self-contained. The term <i>calculus</i> is used here in the sense defined in mathematical logic, where a calculus is treated as an axiomatic theory, the axioms of which are algebraic identities, quasi-identities, or other algebraic forms convenient for calculations. The <i>IC</i> is built upon methods of <i>intensional</i> set theory and is intended to serve as a conceptual toolset for generating, processing, and modifying ideas.</p>