Before scientists began to explore the internal structure of atoms, they were concerned about how and why compounds form. Jöns Jakob Berzelius's dualistic theory, for example, proposed that compounds are formed from atoms because of opposite electrical charges. But this theory did not explain the enormous number of compounds that were formed from four elements alone—carbon, oxygen, hydrogen, and nitrogen. Berzelius called them "organic" compounds because they always seemed to be the products of living beings composed of complex yet highly organized systems. The thinking was that such substances could not be created in the laboratory from inorganic materials, and thus a "vital force"—beyond the understanding of chemists—was necessary to explain their existence.