This foundational statement from Descartes' 'Discourse on the Method' (1637) represents a pivotal moment in Western philosophy. By establishing thought as the irrefutable proof of existence, Descartes presented the cornerstone of modern rational thought. This methodological skepticism revolutionized philosophy by introducing a systematic approach to questioning and understanding reality. The quote continues to epitomize the power of rational inquiry and self-awareness in philosophical discourse, serving as a reminder that conscious thought is our most fundamental connection to existence.