This book claims to inaugurate the thematization of a phenomenology of perception in Heidegger. Running counter to many recent interpretations concerning crucial questions of Heideggerian ontology (particularly the temporal articulation of present and the distinction between Zuhandenheit and Vorhandenheit), it defends the unexpected thesis that the temporality of perception serves as a conductor of the Heideggerian ontology of time, by splitting temporality into authentic and unauthentic modes. Concise commentary on the texts of the Marburg period show that the opposition between the authentic temporality of natural perception (positively identified as circumspection) and the unauthentic temporality of theoretical perception does not merely summarize the entire Heideggerian critique of Husserl but, above all, helps to open a decisive path in the phenomenological account of perception in its evolution from Husserl to Merleau-Ponty.