In this dissertation, I provide an interpretation and defense of Merleau-Ponty's thesis of the primacy of perception, namely, the thesis that all knowledge is founded in perceptual experience. I take as an interpretative and argumentative key Merleau-Ponty's phenomenological conception of motivation. Whereas epistemology has traditionally accepted a dichotomy between reason and natural causality, I show that this dichotomy is not exhaustive of the forms of epistemic grounding. There is a third type of grounding, the one characteristic of the grounding relations (...) found in perception: motivation. I argue that introducing motivation as a form of epistemic grounding allows us to see how Merleau-Ponty's thesis of the primacy of perception avoids both rationalism and empiricism. Whereas empiricism has argued that all the content of our knowledge is grounded in causal interactions between the world and our senses, and rationalism has held that experience does not suffice as a reason for knowledge, thinking of the relation between experience and knowledge in terms of motivation accommodates the best insights of rationalism and empiricism. Motivation allows us to see how knowledge can be grounded in experience without the content of knowledge being reducible to that of experience. (shrink)