Whitehead did not begin his career as a philosopher. In fact, he never had any formal training in philosophy beyond his undergraduate education. Early in his life he showed great interest in and respect for philosophy and metaphysics, but it is evident that he considered himself a rank amateur. In one letter to his friend and former student Bertrand Russell, after discussing whether science aimed to be explanatory or merely descriptive, he wrote: "This further question lands us in the ocean of metaphysic, onto which my profound ignorance of that science forbids me to enter." Ironically, in later life Whitehead would become one of the 20th century's foremost metaphysicians.

What is the highest Whitehead was trained in philosophy?