Answer:

COGITOERGOSUM

Answer	Crossword Clue
COGITOERGOSUM	Cartesian rationale
COGITOERGOSUM	17th-century existence "proof"
COGITOERGOSUM	Philosophical axiom

COGITOERGOSUM	Descartes's statement about pondering an addition problem?
COGITOERGOSUM	Descartes axiom
COGITOERGOSUM	Statement of philosophy
COGITOERGOSUM	Descartes found this truth to be self-evident
COGITOERGOSUM	Self expression?