Answer:

cogito

Answer	Crossword Clue
COGITO	Descartes' hypothesis
COGITO	Philosophical opening
COGITO	Literally, "I think"

COGITO	Descartes opening
COGITO	"... ergo sum"
COGITO	Start of a Descartes declaration
COGITO	... ergo sum (I think, therefore I am): Lat.
COGITO	Start of a line ending in a sum?
COGITO	"Ergo" preceder
COGITO	Start of Descartes' phrase
COGITO	Epistemological principle, familiarly

COGITO	Beginning that leads to a sum?