However, Ehrlich initially believed that receptors existed only for toxins and for physiological foodstuffs and ferments. Drugs and medicines could quite easily be washed out of body tissues with solvents, so that he did not assume that they were fixed to specific components of the cell. He changed his mind only in 1907 [12], partly due to results of his own further research, but in particular also because of a different kind of receptor theory that had been proposed by the Cambridge physiologist John Newport Langley (1852–1925) (Figure 4). Langley's concept is particularly interesting because it placed special importance on the notion of substance and of substance binding.