Earlier quoted context omitted.
I keep making this mistake. I almost asked "why philosophy, didn't he study physics?" That does make me wonder: why DOES the qualification/position/status name mention only philosophy?
PhD is a license to teach (i.e. doctor) philosophy. Specifically, in "science" fields, natural philosophy. Natural philosophy was the original (and i think better) name for science, especially physics. It was understood to be an important subbranch of philosophy. I think its better name because it elucidates an important epistemological difference: science is a method, not knowledge. I learn science when I learn abou…
PhD is the degree you usually get at that level for English, Classics, Economics, Histrory etc..
Some Universities use DPhil (Oxford) for all these but I know of no other degree at that level except for those two (MD is effectively lower and DSc is usually some form of honourary degree and I am not certain what LLD is)