... looked around themselves, not behind ...

The Impressionists as a reaction to the modern world, their modern world. Degas, Monet, Cezanne - three artists with distinct techniques, passions and intentions. Yet they all come under the Impressionist umbrella. It isn't their technique, how they painted which unites them - Monet and Cezanne favoured working outdoors, Degas worked his canvases in the studio. It is what they painted, or what they didn't paint which unites them. It isn't this sort of stuff:

Ingres, Roger freeing Angelica, 1819.

What's that all about?
It's brilliant in it's own way, but what does it tell us of the world that Ingres was living in? Not that there is any imperative for an individual painting to describe the modern condition, but the collective, dominant outlook of painters leading up until the Impressionists took hold was based on the art, the culture, the stories of antiquity, the Bible and the medieval period.
(Forget that the Greeks didn't regard painting as an art at all. Forget that the Bible wasn't exactly keen on images of God.)
The Impressionists looked around themselves and not behind.