The Philosophy of a PhD

Since time immemorial, I have always been wondering why a PhD is called a PhD - Why 'Doctor of Philosophy' and not 'Doctor of Physics/Maths/Chemistry/etc...
It has taken great wisdom from my part to actually type 'Philosophy' in google to really find its meaning. I found 8 meanings - all related to explaining philosophy as a way of thinking leading to wisdom. I give one of it here: "Love and pursuit of wisdom by intellectual means and moral self-discipline".
Not being satisfied by the meaning found through google, I became curious about the etymology of the word and found its origin to two Greek words : 'phil' meaning love and 'sophy' meaning wisdom - but where did 'moral self-discipline' pop in??
So, a PhD, I understand,denotes that the holder of the degree has a love for wisdom and knows how to pursue his love. Still, the meaning of PhD is incomplete without defining what 'wisdom' is!