In his most important philosophical work, the three-volume »Wahrheit and Gewissheit« (»Truth and Certainty«; 1811), the German diplomat and writer Isaac von Sinclair (1775–1815) embarks upon a systematic presentation of human experience. Volume 1 – now edited here – elaborates on the metaphysical and anthropological foundations for human experience. Its point of departure is a discussion of radical doubt, which asks whether man is at all capable of attaining truth and certainty. »How is it possible to distinguish and not-distinguish?« – for Sinclair an encapsulation of the fundamental question posed by doubt per se.