Both Ehrlich and Langley had conceptualised the ‘receptor’ through analogy with chemical binding of substances, and their theories were viewed as ‘chemical’ in nature. This, however, meant that their theories got caught up in the controversy over whether pharmacological substances acted primarily through their chemical or their physical properties. The great competitor to the receptor concepts of Ehrlich and Langley was the so-called ‘potential-poison’ theory of the German pharmacologist Walther Straub (1874–1944).