All great painters are more or less impressionists
In short, a painter may properly be called an impressionist to just the extent that he renders aspects of nature as he saw them with unflinching honesty.
Given its most complete expression in the art of Velasquez, who is still its greatest exemplar, its (impressionism's) aim has ever been to convey on canvas the impression made on the painter by the subject he elects to depict.”
R. H. Ives Gammell
[Velasquez' painting is a] "realism of general aspect."
The realistic or truthful-as-possible depiction of people and things for use in paintings has pretty much always been a goal of representational painters. Pictorial subject matter depicting life as it is lived in reality, rather than its idealized, fantasized, or otherwise altered variant has 'always' been around as well. Romanticism in subject matter, no matter how realistic the drawing and stylization in the work's treatment of objects and people, falls outside the definition of realism.
By the 1600's, the Dutch painters were not only known for their realistic depiction of daily life on the streets, but for actually painting peopled interiors directly from life. Even when depicting historical subject-matter such as Christ healing the sick, Rembrandt had done so realistically (even including a defecating dog in the foreground.) However, it's in Vermeer that we have the Dutch model for the modern impressionist - realist, one who sets up in their studio an entire scene of life just as it might have happened, and then painting it more or less exactly as they see it.
Well, it's not quite that simple, because well before Vermeer something like this form of realism was already being practiced in Spain. Velasquez was raised on it, as evidenced by his early kitchen pictures. Yet from there, it was the direction in which he took this approach which had the greatest early impact on the emergence of realism in the 1800s - particularly with Las Hilanderas and Las Meninas. His fluid, direct-painterly methods and the sense of life they conveyed stood him stand apart from the more typical, hard outlined and carefully modeled works, including his own, leading up to that time. So great was his influence in Boston that recipients of the traveling award at Boston's Museum School were sent to Madrid to copy in the Prado. Their inevitable painting choice, if not their actual assignment, was Velasquez' figure of a woman spinning in the right corner of Las Hilanderas.
In that same Museum School training, directed as it was by Antwerp trained painter, Otto Grundman, Dutch painters were also extolled as examples for emulation. Typically great Dutch craftsmanship would be pointed out, yet with the advent of a new emphasis on light and color following the advent of Monet. The master of light, Jan Vermeer, became a painter of particular interest to those who would subsequently lead the Boston School. It isn't surprising that Museum School trained (and eventual teacher there) Philip Hale, would select Vermeer as the subject for a book. Hale's thesis, not only a singular written presentation of Vermeer's art, does a remarkable job in also introducing key tenets of Boston School impressionism. It should be read by anyone wishing to understand them. Paxton, who collaborated in Hale's research and analysis, is reported to have extolled Vermeer as his ideal of good painting - something Gammell clearly bought into as well. At times Paxton seems to be almost self-consciously imitating his work. As with Velasquez, graduates of the Museum School often copied Vermeer in Antwerp or elsewhere to complete their training
As indicated, many of the Dutch were occupied with the idea of painting things just as seen; in the room, or on the table, before them. Still lifes, interiors, or genre scenes were most often set up and copied accurately, granting further impetus to the observation of visual truths. With some exceptions (Rembrandt being the most notable) conventions related to painting remained fairly uniform - dominated by the idea of drawing the figure or object pre-eminent, with its outlines executed first, and then its interior surface modeled with mid-tones rather like a modern day coloring book. With their finely articulated two-dimensional accuracy, and their continuous lines, Holbein's drawings often look as if traced on a glass. When Vermeer paints interiors, as in The Astronomer, he outlines each object in the same way. Even he is clearly operating from object outlines.
Yet in Velasquez, who is obviously more interested in the larger visual impression for its own sake than Rembrandt, we see a real shift in the approach from the old way of painting. Velasquez's goal is to articulate the visual world as it appears, on its own terms, even when it includes an entire ensemble of people, and a variety of spaces. To execute such a complicated picture in a life-like way, he found new efficiencies: ways of getting more quickly to the point, and to the collection of relationships making up the big visual impression as we would call it today. His orientation is similar in certain ways to that of a French still-life artist named Jean Simeon Chardin, the other painter of key interest to Tarbell and the Boston school.
Carolus-Duran and Dannat (the latter of which would have a major influence on Tarbell and Benson) as well as numerous others of their generation had found Velasquez' later approach well suited to their realist/impressionist ambitions. In the 1800's, some say as a result of the French Revolution of 1848 or even earlier, a realism understood as getting back to the depiction of life as it really is, began to dominate the visual arts. Sargent, who had adopted Carolus-Duran's approach almost exactly, had spent time in Spain working out his version of Velasquez' impressionism. Meanwhile Sorolla, Zorn, and our friends which made up the Boston School among many others dropped, for example, indirect and outline-first academic painting even when making imaginative pictures. Sargent's Oyster Gatherers of Calais and Smoke of Ambergris are examples.
Yet even before that particular "French Revolution," the Dutch had already been developing painting along similar lines. Eventually, or one is tempted to say inevitably given their strong background in painting directly from life, landscape painting on location came to be attempted there. By the early 1800s, it was spreading south. At sixteen years of age, Monet is said to have found a Dutch trained painter, Boudin, painting on the beach. He put himself under his mentorship, eventually working with the artist's teacher, Jongkind - all in pursuit of the fascinating art of plein aire painting. Their approach to painting was also nearly outline free. By the time we get to Monet in particular, as he digs deeper into the study of the color of light directly before nature, it is clear that ever more effective and efficient processes for execution finally became de rigeur. At that point the methods of painting from life changed decidedly. And those of the Boston School were one of the more impressive in achieving all impressionism could hope for, what Gammell would go on to call the 'greatest evolution of impressionism.”