Original documents by L. P. Hammett and C. K. Ingold are presented which elucidate the origin of physical organic chemistry in the late 1920s and early 1930s. The field gained fast popularity, and within four decades created the presently accepted mechanistic model of organic chemistry. As the focus of research in organic chemistry shifted to synthesis around 1970, physical organic chemistry lost visibility, though mechanistic investigations became integral aspects of synthetic method development. While important subdisciplines of physical organic chemistry, e.g., supramolecular chemistry or mechanistic enzymology, continued to develop under separate headings, the systematic use of newly developed techniques (gas phase chemistry, laser spectroscopy, matrix isolation) is expected to lead to a renaissance of classical physical organic chemistry.