WILLIAMSTOWN — Of all the Impressionists, Camille Pissarro was the most sympathetic. His name has never attained the luster of Monet, Degas, Renoir, Cézanne, van Gogh, Gauguin, or Seurat. And yet if you plotted all these Impressionists and Post-Impressionists on a Venn diagram, Pissarro would be the most frequent point of overlap.

Not only was he one of the prime movers in the formation of the breakaway Impressionist group (he contributed to all eight of the Impressionist exhibitions, held between 1874 and 1886), but he was also a friend and adviser to younger colleagues, including Gauguin, Seurat, and Cézanne.

In his art, as in his intellectual life, Pissarro was curious, hungry, open to influence. His letters to his eldest son, Lucien, stand beside van Gogh’s letters and Delacroix’s journals as among the great documents of 19th-century art. In these letters, as in his pictures, Pissarro comes across as integrity incarnate.

The problem he poses for a critic is that, although he painted a steady stream of very good pictures, he never really painted a masterpiece. Can one be a great artist without painting masterpieces? The short, honest answer is no. But you can be an exemplary artist, and Pissarro was that.

If you don’t know Pissarro, the Sterling and Francine Clark Art Institute’s wonderful summer show “Pissarro’s People’’ is a fine introduction. If you do know him, it will make you feel you didn’t.

The exhibition is the first serious attempt to focus on Pissarro’s family pictures and figure paintings. If, as a result, we miss out on the dew-kissed, mint-fresh landscapes or the late, magisterial cityscapes for which he is best known, the windfall is that we get a much stronger sense of what he was like as a man — what mattered to him.

What mattered most was family. The show, which is organized by theme, is particularly rich in Pissarro’s portraits of his children.

A famous portrait of his third son, Félix, shows the boy in three-quarter profile against green patterned wallpaper. He wears a red beret, which chimes with his rosy cheek, his plump lower lip, and the pink scarf tied in a bow at his neck.

He is, in short, adorable.

His father clearly delighted in the boy’s loose brown locks, which come to a curling halt on his shoulders, and in his blue eyes. Dilated with trepidation, they arouse one’s protective impulses as surely as a playful kitten incites delight.