Let us say there are two artists sitting at a table.
One artist has a coloring book. He opens up his book and finds a drawing of a barn and some farm animals. With only a small box of crayons, this artist does not have a lot of options. You can imagine that most people in his position would make the sky blue, the sun yellow, the cows white and black, the grass green, the barn red. This kind of coloring is mechanical. With those same crayons, you can make a robot draw the same colored picture, and no one would know the difference.
The second artist has the same coloring book, but instead of crayons, he has a full color palette of oil paints. He can mix and adjust colors to whatever he wants. But adjust them to what? Instead of deciding for himself that the sky is blue and the grass is green, this artist goes outside to a nearby farm to adjust his colors to those of Nature. With the full color palette at his disposal, this artist studies the subjects in front of him very carefully before putting down his colors. The barn gets some red here, a little blue there, some yellow over there. These colors are surprising to the artist, but that is what he sees, and what he sees is true to Nature. And when what he has painted looks like what he sees, this artist can be said to be painting not mechanically, but optically.
He is using his own powers of observation to make a painting that no robot nor any other artist could replicate, because they do not see the same way. You can line up Rembrandt, Raphael, and Monet next to this farm, and ask them to paint it. They are each very careful observers of Nature, but even with the same subject and the same color palette, they will produce surprisingly different paintings. They would undoubtedly be curious to study each-other’s paintings because of these differences. This is what aspiring artists do when they go to visit museums–to see how Monet painted that church, how Rembrandt painted that self-portrait, how Raphael painted that woman. The greatest artists are the most inimitable artists, because their powers of observation are so great. You can easily identify paintings not done from Nature by not being surprised by the color choices when you walk up to them. Contours and forms are also surprising when closely observed from Nature. It is best to trust exactly what your eyes see, and to not put down what you think you see. Artistic techniques are improvised and experimental methods used to make the artwork more true to Nature.
Again, if you are wondering why artists couldn’t be replaced by robots, it is because the greatest artists create work that is so wildly eccentric that they are likely to be one of a kind for all of eternity. Meanwhile, legions of mediocre artists are all doing things that robots could easily be programmed to produce. And nowadays, you do see people programming robots to simulate mediocre art. But nothing great. This is why I hate to see mediocrity in museums. It makes people forget that the great masterpieces of the world are only great because the artists that created them were the most sensitive observers of Nature, and had the ingenuity and craftsmanship to forge works of incredible beauty and power. It is the kind of ability that is best done by humans, for humans. I honestly believe that the vast majority of people live in a sort of Matrix, and it is the great artists that let us know through their works that the human experience can, and should be, boundless.
If I had seven peasants, I could make seven lords but if I had seven lords, I could not make one Holbein — Henry VIII to Sir Richard Tavistock when he complained about Holbein painting his nude fiance, Lady Ursula.
A note on mechanical reproductions: they can be valuable substitutes in the same way that zoos and aquariums are valuable to city dwellers, but the real majesty is in diving deep into the ocean or into the rainforest. A real work of art in its intended environment or frame has that same majesty–the hand of the artist is right there in front of you, and you are only a visitor into their grand world. Mechanical reproductions attempt to bring some of that grandeur to you, like pulling a Mahi-mahi out of the ocean, but it quickly loses its luster.
Also see “Philippe de Montebello on the Museum vs. the iPad”: https://www.youtube.com/watch?v=83FfXzOca1E