At the time the approved and accepted method of painting was in the Renaissance grand manner of Leonardo, Michelangelo and Raphael, which have been exemplified by Nicolas Poussin who died in 1665, among others.
Draughtsmanship was all, art was about precision, a pallet of earthy tonal colors were to be blended and applied to the canvas with precise brushstrokes, that over many many hours and days of work could be honed to the point of imperceptibility.
Through subtle gradations between light and shade, the objective was to produce a painting that gave the appearance of three-dimensional solidity.
Which was fine, when sitting in a warm room for weeks on end, elaborately rendering a dramatise scene. But the Impressionists were outside painting in the open air.
The light they worked in changed constantly. If they were to paint with any grain of truth, speed was now called for.
No time to dwell on labourious gradations of light because the next time the artist looked up it would’ve changed. Instead, urgent rough sketchily applied brushstrokes replaced the studied blended refinement of the grand manner.
Brushstrokes that the Impressionists made no attempt to hide, quite the opposite, they accentuated their brush brushwork by painting short colorful-like bursts which added a sense of youthful energy to their paintings, reflecting the spirit of their age.