Conceive of a picture really as a series of harmonies.

Melody and harmony are like lines and colors in pictures. A simple linear picture may be completely beautiful; the introduction of color may make it vague and insignificant. Yet color may, by combination with lines, create great pictures, so long as it does not smother and destroy their value.

Well! according to the sketch your picture [which Whistler sent him in a letter] seems well-composed, the lines of sky, sea, the position of the figures all that seems very, very well done[,] on opening the letter it struck me immediately, an impression of decorative ornamentation which makes a good picture appeal to the eye as well - upside down and any other way up. Arrangement, layout, composition. etc - mysterious words harmonious laws - in no way conventional - needs of the Artist glory of the Raphael's Michelangelo's etc etc - for me a repetition a hesitation, a matter of feeling - which should also be a law, a mathematical thing, like form, like light, colour[.] I find similarities with the colour method which works by opposing colours to arrive at a harmony that is to say to make a canvas a complete whole, to put into a small space an image with all the forces all the principles of nature - [instead of showing a part of it]..

Aesthetic:
Art is Harmony.
Harmony is the analogy of opposites, the analogy of similarities of tone, of tint, of line taking account of a dominant and under the influence of the lighting, in combinations that are gay calm or sad.

I borrow some subject or other from life or from nature, and, using it as a pretext, I arrange lines and colors so as to obtain symphonies, harmonies that do not represent a thing that is real, in the vulgar sense of the word, and do not directly express any idea, but are supposed to make you think the way music is supposed to make you think, unaided by ideas or images, simply through the mysterious affinities that exist between our brains and such arrangements of colors and lines.

As music is the poetry of sound, so is painting the poetry of sight, and the subject-matter has nothing to do with harmony of sound or of colour. The great musicians knew this. Beethoven and the rest wrote music — simply music; symphony in this key, concerto or sonata in that.. .Art should be independent of all claptrap — should stand alone, and appeal to the artistic sense of eye or ear, without confounding this with emotions entirely foreign to it, as devotion, pity, love, patriotism, and the like. All these have no kind of concern with it; and that is why I insist on calling my works 'arrangements' and 'harmonies.'

At the moment there is a great tendency in painting to discover the 'new' harmony by constructive means whereby the rhythmic is built on an almost geometric form.. .I am certain that our own modern harmony is not to be found in the 'geometric' way, but rather in the anti-geometric, anti-logical way. And this way is that of 'dissonance in art', in painting, therefore, just as much as in music. And 'today's' dissonance in painting and music is merely the consonance of 'tomorrow'.

There is a musicke where-ever there is a harmony, order or proportion; and thus farre we may maintain the musick of the spheares; for those well ordered motions, and regular paces, though they give no sound unto the care, yet to the understanding they strike a note most full of harmony. Whatever is harmonically composed delights in harmony; which makes me much distrust the symmetry of those heads which declaime against all Church musicke... Even that vulgar and Taverne Musicke, which makes one man merry, another mad, strikes in mee a deepe fit of devotion, and a profound contemplation of the first Composer; there is something in it of Divinity more than the eare discovers. It is an Hieroglyphicall and shadowed lesson of the whole world, and Creatures of God, such a melody to the eare, as the whole world well understood, would afford the understanding. In briefe it is a sensible fit of that Harmony, which intellectually sounds in the eares of God.

There is a musicke where-ever there is a harmony, order or proportion; and thus farre we may maintain the musick of the spheares; for those well ordered motions, and regular paces, though they give no sound unto the era, yet to the understanding they strike a note most full of harmony. Whatever is harmonically composed delights in harmony; which makes me much distrust the symmetry of those heads which declaime against all Church musicke. For my self, not only from my obedience but my particular genius, I doe embrace it; for even that vulgar and Taverne Musicke, which makes one man merry, another mad, strikes in mee a deepe fit of devotion, and a profound contemplation of the first composer; there is something in it of Divinity more than the eare discovers. It is an Hieroglyphicall and shadowed lesson of the whole world, and Creatures of God, such a melody to the eare, as the whole world well understood, would afford the understanding. In briefe it is a sensible fit of that Harmony, which intellectually sounds in the eares of God.

There is a musicke where-ever there is a harmony, order or proportion; and thus farre we may maintain the musick of the spheares; for those well ordered motions, and regular paces, though they give no sound unto the eare, yet to the understanding they strike a note most full of harmony. Whatever is harmonically composed delights in harmony; which makes me much distrust the symmetry of those heads which declaime against all Church musicke. For my self, not only from my obedience but my particular genius, I doe embrace it; for even that vulgar and Taverne Musicke, which makes one man merry, another mad, strikes in mee a deepe fit of devotion, and a profound contemplation of the first composer; there is something in it of Divinity more than the eare discovers. It is an Hieroglyphicall and shadowed lesson of the whole world, and Creatures of God, such a melody to the eare, as the whole world well understood, would afford the understanding. In briefe it is a sensible fit of that Harmony, which intellectually sounds in the eares of God.

Formerly it frequently happened to me that when questioned regarding a picture I simply did not know what it represented. I had not seen the subject, so to say. Now I have also included the content so that I know most of the time what is represented. But this only supports my experience that what matters in the ultimate end is the abstract meaning of harmonization (note from a letter, 1903)

Philosophers and aestheticians may offer elegant and profound definitions of art and beauty, but for the painter they are all summed up in this phrase: To create a harmony

[another part / version of Whistler's lecture:]
Nature contains the elements, in colour and form, of all pictures, as the keyboard contains the notes of all music. But the artist is born to pick, and choose, and group with science, these elements, that the result may be beautiful—as the musician gathers his notes, and forms his chords, until he bring forth from chaos glorious harmony. To say to the painter, that Nature is to be taken as she is, is to say to the player, that he may sit on the piano. That Nature is always right, is an assertion, artistically, as untrue, as it is one whose truth is universally taken for granted. Nature is very rarely right, to such an extent even, that it might almost be said that Nature is usually wrong: that is to say, the condition of things that shall bring about the perfection of harmony worthy a picture is rare, and not common at all.

We have never heard Seurat, Cross, Luce, Van de Velde or indeed Van Rysselberghe or Angrand speak of dots. We have never seen them be preoccupied by Pointillism. Read these lines, dictated by Seurat to Jules Christophe, his biographer: 'Art is harmony; harmony is the analogy between opposites and between similar elements of tone, tint and line. By tone I mean light and dark; tint is red and its complementary: green, orange and its complementary: blue, yellow and its complementary: purple.. .The method of expression relies on the optical mixture of tones, tints and their reactions (shadows that follow very strict rules).