"What I am trying to reproduce for you is more intangible, it is intertwined with the roots of being itself, at the intangible source of sensation."  (Paul Cézanne)
The source of sensation of which Cézanne writes is always present in us – in every moment that is entirely our own.
It can happen then that, after coming from a painting exhibition in a museum, I suddenly perceive my surroundings in a way that is unusual to me. At first, the stones on the floor catch my attention. I see colours and nuances of colour on the stones, which must have been left on them by weather conditions and human imprints. I am completely immersed in what I perceive there and am in unity with the colours and the contours. I feel absorbed in a magical world of shades, shapes and scents at the root of my sensations.
The French phenomenologist Maurice Merleau-Ponty explored these sources of our sensations by researching the nature of human perception. Maurice Merleau-Ponty (MP) was one of the most famous philosophers of the 20th century. "If phenomenology found its own stature in France quite soon, it is in particular thanks to Merleau-Ponty." 
"Phenomenology, what is it?"
MP begins the preface to his major work Phenomenology of Perception  with this question. The term "phenomenon" is derived from the Greek phainomenon, meaning something appearing or a visible event.
Plato distinguished between changeable phenomena and eternal ideas. Phenomenology is a philosophical current of the 20th century, founded by E. Husserl and M. Heidegger, who wanted to return to phenomena, to "the things themselves" and the structure of the experiences of the human life-world.
Phenomenology is founded on a method that does not empirically, analytically or reflexively intellectually investigate a phenomenon, that is, a sensory experience, but searches for its essence and describes its traces.
"To go back to the 'things themselves' is to go back to this world that precedes all cognition, of which all cognition speaks and in relation to which all determination and science necessarily remain abstract, signifying secondary, just as geography is to landscape, in which we first of all learned what such things as forest, meadow and river actually are." 
Empiricism and intellectualism
MP developed his own phenomenological method by distancing himself procedurally from the handed-down traditions of thought of experimental empiricism, on the one hand, and intellectualism, on the other.
The empirical method forces perception into causal thinking and thereby reduces it to a sensory-physiological stimulus-response scheme.
Intellectualism, on the other hand, assumes a subject consciousness that constructs the world; it establishes scientific theories and shapes the world as it perceives it to fit them. Descartes, for example, in his famous sentence Cogito ergo sum, bases his existence on the self-consciousness of his thinking.
MP says: "It is not that the „I-think“ eminently contains the „I-am“, it is not that my existence is reduced to the consciousness I possess of it; rather, conversely, the I-think finds itself integrated in [...] the I-am of its existence: The I-am precedes thinking."
Scientific thought thus creates its own reality. In both schools of thought there is a separation between the subject and the object of perception. There is a determination to treat all being as a thing par excellence ... as if it were predestined for our manipulations. "Science experiments with things and refrains from being with them." 
MP therefore calls this thinking a "thinking that flies over", a thinking in overflight, that is, a thinking that is not rooted in the nature from which it comes.
The empirical method and empiricism skip over the fact that perception can only take place in a "body".
The centrality of the human subtle body
The human subtle body is central to MP's philosophy. It mediates between body, soul and spirit and thus enables the human being to be an incarnate being. Since the subtle body, as a fact, is itself imperceptible, it represents the missing link in the perceptual processes of the traditions of thought discussed.
"Perception cannot be described as one among the facts that occur in the world, since we [...] are never able to suppress that empty place that we ourselves are [...] and [...] perception is the 'flaw' in this 'great gem'".
According to MP, the subtle body is a texture of an element he describes as ether, which underlies all appearances in different modalities of being. He says: "One's own body is in the world like the heart in the organism: it is it that keeps all visible spectacle ceaselessly alive, inwardly nourishing and animating it, forming with it a single system."  It is the expression of animate life and space-generating movement; it opens us to the world and communicates with it in the sensation in which our perception is rooted.  The subtle body is also the synthesis of all parts of the body whose senses are rooted in it; it is, so to speak, a "natural I".
"The theory of the body schema is implicite already a theory of perception" , he concludes.
The birth of perception from sensation
"Art, and painting in particular, draw from that sea of raw sense of which productive thought wants to know nothing. They are even the only ones who do this in all innocence. [...] Only the painter has the right to cast his gaze on all things without being obliged to judge them. Before him, one might say, the ordering concepts of knowledge and action lose their efficacy." 
MP had a long inner dialogue with the painter Paul Cézanne about his process of perception, which we want to trace here in individual aspects.
Cézanne suffered constantly from self-doubt; he was only recognised at a late stage and then became a role model for subsequent generations of artists, however, because they saw the whole secret of modern painting hidden in every single one of his paintings. Matisse even called him "a kind of dear God of painting".
Cézanne loved to paint in the open air because he could move freely in it.
MP: "He thus brings in his subtle body", and by "... lending his sublte body to the world, he transforms the world into painting [...], and to understand this, one must recover the [...] present subtle body, [...] which is a mesh of vision and movement." 
In the midst of the landscape, Cézanne created his optics, but "by optics I understand a logical seeing, that is, not something contrary to reason", he said.
He saw the whole field of perception before him, that is, the great horizon of the landscape, the totality of its fullness, and by first obtaining clarity about its geological structures, he worked out his field of vision. He let the landscape germinate within him and found his motif, a still invisible figure. Then he stopped moving and only looked "until his eyes", as Madame Cézanne said, "popped out of his head".
MP explains that Cézanne lets us participate "in a process of perception that gives us a 'logos in statu nascendi' beyond any dogmatism under real conditions of objectivity ..."  Cézanne did not paint his perspectives according to the textbooks. Thus, at first they appear somewhat rigid on the canvas, but if you look more closely, you see that our eye complements their spontaneous movement. Cézanne painted only according to what he experienced as visible. MP explains that psychology discovered late that the perspective we experience is not that of geometric perspective.
For this reason, Cézanne did not paint fixed contours around objects. His famous apples do not find their shape through bounding lines. He followed their curving edges through colour modulations that make them appear round and plump, just as our eye, which is not a photographic one, perceives them. It is not a matter of reducing human knowledge to sensations, but of witnessing the birth of that knowledge. MP explains that in primordial perception there is not yet any difference between the senses. "It is only the science of the human body that later teaches us to distinguish between our senses." 
Rather, Cézanne experienced his motive as a centre from which the sensory data radiate.
"The world is what we perceive," MP says. 
Our perception of the world is based on a fundamental relationship of body, soul body and spirit. "For me, this perception lies in sensation," Cézanne explained. Sensation, in turn, takes place in and through the subtle body, for it gathers within itself all the senses that strive for expression of their underlying "logical" unity. "Rather, the sensory properties of a thing constitute in one and together a same thing, just as my gaze, my feeling and all my faculties constitute one and the same, that is, the body integrated in its unified action." 
Invisibility within visibility
Man stands on a double ground for, on the one hand, he stands in a visible world and, on the other, in an invisible world.
He has a visible physical body which is enlivened and animated by an invisible body.
In the objective thinking of understanding and science, which eludes primordial experience, a consciousness is formed that objectively posits a phenomenon, such as our body. However, "[...] the very positing of a single object is the death of consciousness, since, like a solution crystallises entirely by the introduction of a single crystal, it congeals all experience [...].
If we succeed in piercing this thinking [,..] this should be the decisive turning point. We will see how, in science itself, the subtle self-body eludes the treatment to which it wants to subject it." 
It is again art that teaches us the mystery and depth of perception
A painted image is itself invisible, but it makes visible.
Cézanne sometimes paints people with faces like objects, reproducing the external anonymous reality of his time, but if we look more closely, "the spirit becomes visible and readable in the looks, which are themselves nothing but complexes of colour. [...] The painter who thinks and heads directly for expression misses the mystery of the sudden appearance of a human being in nature, which is always renewed as soon as we catch sight of a human being."  The painted image itself shows us an impenetrable face, yet when we delve into the silent face, it reveals its secret to us in its expression.
"It is as if every part knew of all," Rilke said of Cézanne's paintings.
The same is true of the spoken word; its background is the silence that can express its mute meaning.
It is again the subtle body that gives meaning here as a hidden unit of expression, because art is an act of expression.
MP suggests that the philosopher, too, (and we can add the scientist, too,) should "engage with the silent given world in order to help it express itself from the depths of silence".
In this permeability of the planes that connect everything with everything, in which the subtle body itself belongs to an invisible world-spanning matrix, MP sees a future thinking and a new being of man.
"That which exists is an infinite task." (Maurice Merleau Ponty)
 Maurice Merleau-Ponty, Das Auge und der Geist, Philosophische Essays, Hamburg 2003, p. 275
 Bernhard Waldenfels, Phänomenologie der Wahrnehmung in Frankreich, Frankfurt 1987, p. 142
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 Maurice Merleau-Ponty, Das Auge und der Geist, a.a.O., p. XIV (Einleitung)
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