The belief dating to around 1600 that matter was divided into two classes based on behavior with respect to heat: organic and inorganic. Inorganic material could be melted but could always be recovered by removing the heat source. Organic compounds changed form upon heating and could not be recovered by removing the heat source. The proposed explanation for the difference between organic and inorganic compounds was the Vitalism Theory, which stated that inorganic materials did not contain the "vital force" of life and lasted until the mid-nineteenth century.