In his investigation of the Phenomenology of Perception (1945), Maurice Merleau-Ponty defines phenomenology as the study of essences, including the essence of perception and of consciousness. He also says, however, that phenomenology is a method of describing the nature of our perceptual contact with the world. Phenomenology is concerned with providing a direct description of human experience.

Perception is the background of experience which guides every conscious action. The world is a field for perception, and human consciousness assigns meaning to the world. We cannot separate ourselves from our perceptions of the world.

Merleau-Ponty argues that both traditional Empiricism and Rationalism are inadequate to describe the phenomenology of perception. Empiricism maintains that experience is the primary source of knowledge, and that knowledge is derived from sensory perceptions. Rationalism maintains that reason is the primary source of knowledge, and that knowledge does not depend on sensory perceptions. Merleau-Ponty says that traditional Empiricism does not explain how the nature of consciousness determines our perceptions, while Rationalism does not explain how the nature of our perceptions determines consciousness.

Perception may be structured by associative forces, and may be focused by attention. Attention itself does not create any perceptions, but may be directed toward any aspect of a perceptual field. Attention can enable conscious perceptions to be structured by reflecting upon them.

Merleau-Ponty explains that a judgment may be defined as a perception of a relationship between any objects of perception. A judgment may be a logical interpretation of the signs presented by sensory perceptions. But judgment is neither a purely logical activity, nor a purely sensory activity. Judgments may transcend both reason and experience.

Perception is not purely sensation, nor is it purely interpretation. Consciousness is a process that includes sensing as well as reasoning.

Experience may be reflective or unreflective. Unreflective experience may be known by subsequent reflection. Reflection may be aware of itself as an experience. Reflection may also be a way to understand and to structure experience.

Reflection may be focused successively on different parts of a perceptual field. According to Merleau-Ponty, perceptual objects have an inner horizon in consciousness and an outer horizon in the external world. The object-horizon structure enables the individual to distinguish perceptual objects from each other. All objects reflect each other in time and space.

Psychological and physiological aspects of perception may overlap and influence each other. The spatiality of the human body, or the 'body image,' is an example of how both psychological and physiological factors may influence perception.

Perception is a system of meanings by which a phenomenal object is recognized. The intentions of the person who is perceiving an object are reflected in the field to which the phenomenal object belongs. Merleau-Ponty argues that consciousness is not merely a representative function or a power of signification. Consciousness is a projective activity, which develops sensory data beyond their own specific significance and uses them for the expression of spontaneous action.

According to Merleau-Ponty, the human body is an expressive space which contributes to the significance of personal actions. The body is also the origin of expressive movement, and is a medium for perception of the world. Bodily experience gives perception a meaning beyond that established simply by thought. Thus, Descartes’ cogito ("I think, therefore I am") does not account for how consciousness is influenced by the spatiality of a person’s own body.

Merleau-Ponty also argues that existence and substance presuppose each other. Substance expresses existence, and existence realizes itself through substance. However, substance is not merely a form of signification or expression of existence, and existence is not merely what is expressed as substance. Existence and substance explain each other.

Merleau-Ponty says that thought precedes speech, in that speech is a way of expressing thought. Thoughts which cannot be expressed are temporarily unconscious. Thoughts which can be expressed can become conscious. Whether or not thoughts can become conscious may depend on whether or not they can be expressed. But we can become conscious of thoughts even if they have not previously been expressed.

Speech can express the thoughts of the person who is speaking, and the listener can receive thoughts from the sounds of spoken words. Thoughts may exist through speech, and speech may be the external existence of thought. But speech is not merely the expression of thought, because speech may have a power of signification of its own.

Existence is a condition that includes the existence of conscious beings and of nonconscious things. Bodily experience is an ambiguous mode of existence, because the idea of the body cannot be separated from the experience of the body, and because mind and body cannot be separated as subject and object. The mind and body each have their own being, and the perceptions of the body influence what is perceived by the mind.

Every sensation belongs to a sensory field. The concept of a sensory field implies that all senses are spatial, and that all sensory objects must occupy space. Every object which is perceived belongs to a field of other objects which are not perceived. Every perceived sensation belongs to a field of other sensations which are not simultaneously perceived by the subject.

Space may be defined as a form of external experience, rather than as a physical setting in which external objects are arranged. The relationships between objects in space are revealed by the experience of the perceiving subject. A perceptual field is a field in which perceptions are present in time and space. Space is modified and restructured by time.

Memory is a capacity to recall or recognize the past, and may be influenced by changes in perceptions. Perceptions may be true or false. An illusion may be a false perception, or a perception of something in an unreal way. A hallucination may be a perception for which there is no causative stimulus in the external world.

Merleau-Ponty argues that consciousness is transparent in that it is not concealed from itself. The unconscious may be concealed from the conscious, but the conscious can be revealed to itself. Both appearance and reality are phenomena of consciousness. Appearances may be true or false, and may or may not be the same as reality. The false appearance of a perceptual object may conceal its true reality. However, the actual appearance of a perceptual object may also manifest the object’s true reality. Thus, phenomenology is concerned both with appearance as a perceptual phenomenon, and with reality as a perceptual phenomenon.

Merleau-Ponty concludes by defining freedom as a mode of consciousness in which personal actions and commitments can be chosen within a situation or field of possibility. Freedom is always within a given field of possibility. Freedom is always present in a situation, unless we lose our belonging to the situation. Freedom is a mode of being-in-the-world which enables us to transcend ourselves.