This text analyzes the relationship between Cartesian self-certainty and human mortality through Eugen Fink's interpretation. The author builds on Descartes' concept of the 'fundamentum inconcussum,' asserting that while external objects may be subject to sensory deception, the existence of the experiencing subject remains indubitable. Even a malicious spirit cannot deceive an individual about their own being while they perceive themselves. However, the text argues that this unshakable self-assurance harbors an inherent certainty of death, described as the 'worm of mortality.' Human existence is not absolute or divine but is fundamentally defined by its finitude. The certainty of 'I am' is thus paradoxically intertwined with the awareness of a future 'I will not be.' Deception is viewed as a mild form of nothingness, whereas death represents nothingness in its most radical form. Fink’s reflection expands the Cartesian framework by adding an existential dimension, where self-awareness is inseparable from the consciousness of one’s own temporal limitation and eventual non-existence.