Cézanne retained the fundamental doctrine of painting from nature but with added rigour, famously saying ‘I want to re-do Poussin from nature’. (Poussin being a notoriously intellectual pioneer of French landscape). Seurat put impressionist painting of light and colour on a scientific basis (neo-Impressionism, divisionism). Gauguin retained intense light and colour but rejected painting from nature and reintroduced imaginative subject matter. Van Gogh painted from nature but developed highly personal use of colour and brushwork directly expressing emotional response to subject and his inner world.
Post-impressionism is a term which describes the changes in impressionism from about 1886, the date of last Impressionist group show in Paris