This paper focuses on the concept of the "intentional arc" in Merleau-Ponty, who maintains that perception comes into play within, and is nourished by, an already established relation between the person and the world. That obscure relation, the intentional arc, is the "genesis" of perception, and this paper argues that in it resides the proper theme of a psychological phenomenology of perception. A study of the intentional arc shows that perception is not a passive, causal, impersonal process. On the contrary, perception is active in that it requires the perceiver's free consent to the solicitations stemming from the world; it is thus only motivated or beckoned, but not caused; and it rests on a personal foundation, which is to say that it depends on the entire psychological life of the perceiver. A subsequent paper will enrich and develop the implications of this thesis by taking up the phenomenology of hallucinations.