Maurice Merleau-Ponty’s (1908–1961) main philosophical work, Phenomenology of Perception, is known for its detailed argument for the claim that perception, the locus of evidence and truth, is embodied. A central part of the argument is the analysis of the anonymous bodily life at the basis of perception. An important but less noted claim endorsed by Merleau-Ponty in his analysis of the infrastructure of perception is that perception rests upon instinctive operations of the perceiver’s body. The thesis uncovers Merleau-Ponty’s thinking of the instinctive groundlayer of perceptual experiencing. It takes as the leading clue the term biological existence by which Merleau-Ponty refers to the instinctive operations of the body in Phenomenology of Perception. The thesis shows that even if the term has a scientific connotation, Merleau-Ponty’s purpose is not to argue that a life scientific conception of the body would reveal the ground of perception. Instead, scientific descriptions of the body, notably the ethological characterization of instinctive behavior, will ultimately provide means for delimiting and describing a primitive mode of intentional lived experiencing. The thesis consists of seven chapters. Chapter 1 is an explication of the philosophical context of Merleau-Ponty’s early works, The Structure of Behavior and Phenomenology of Perception, and an explication of Merleau-Ponty’s understanding of the basic tenets of phenomenology. Chapters 2 and 3 sort out Merleau- Ponty’s phenomenological interpretation of the scientific studies of behavior, his conception of the organism, and the primitive behaviors of instinct and reflex. Chapters 4, 5, 6 and 7 elaborate a variety of aspects of instinctive experiencing in Merleau-Ponty’s phenomenology of perception. The work explicates and discusses the instinctive mode of temporality, motor intentionality, affectivity and sensibility, and it shows that a coherent line of thought is opened up by the question of biological existence in Merleau-Ponty’s Phenomenology of Perception. Keywords: Merleau-Ponty, perception, instinct