On July 16, 1948, Experimentele Groep was founded by Karel Appel, Corneille, Theo Wolvecamp, Constant, Jan Nieuwehuys, Anton Rooskens and Eugene Brands. Later, this group became known as the Dutch core of the international CoBrA movement. The artists had a close collaboration with experimental groups, e.g. from Paris, Copenhagen and Brussels.

 Experimentele Groep published a magazine called Reflex, a name that referred to the fact that they saw their works as an immediate response to reality. The manifesto was later seen as one of the most important texts on art in the Netherlands, shortly after the Second World War. In the manifesto, Constant wrote that the process of creation is more important to the experimental artist than the resulting work. It is a means of attaining spiritual and mental wealth. Second, the work of an experimental artist is a reflection of the changes in the general perception of beauty.