The crossword solutions for the term CARTESIAN 'I AM' is SUM, which stands for the Latin phrase 'SUM res cogitans', meaning 'I am a thinking thing'. This phrase was coined by the French philosopher and mathematician Rene Descartes in his 1641 work, Meditations on First Philosophy. Descartes believed that the phrase was a fundamental truth of human existence, and it has since become a cornerstone of modern philosophy.