Final answer:
Heidegger identified a major source of anxiety as the encounter with existential uncertainty and the lack of absolute meaning in life's foundations.
Step-by-step explanation:
Martin Heidegger, a central figure in existentialist philosophy, suggested that a major source of anxiety was the loss of certainty in life's foundations. This view can be linked to Friedrich Nietzsche's proclamation that "God is dead," which represents a departure from objective truth and the anxiety of facing a world devoid of absolute meaning. Contradictory beliefs and the realization that previously held 'absolute' truths are no longer tenable can be profoundly unsettling, resulting in existential anxiety.
In considering existentialism, the immense burden of personal freedom and the lack of absolute ethical mandates also contribute to anxiety. Heidegger's phenomenological method, emphasizing the direct investigation of existence within the world rather than abstract speculation, and the importance placed on individual interpretation over universal meaning, underscores this anxiety. It is the encounter with the 'abyss'—the realization that meaning is not predetermined, but is instead constructed by individuals—that imparts a genuine sense of unease, grappling with the inherent existential uncertainty of life.