Abstract

At first sight one might be tempted to regard Descartes' "cogito ergo sum" as logically true by existential generalisation. This however would neither exhaust the specific epistemic content of "cogito" nor reveal the philosophical peculiarities of "sum" which the author takes to have two ontologically different meanings. The full sense of "cogito ergo sum" finally turns out to be "Credo* me* cogitare ergo scio* me* $\text{esse}_{1/2}$ ". Furthermore this proposition can formally be proved to be true by means of epistemic logic.