Didascalicon, Book III: the art of study (discipline and method)
Chapter V. On the coherence of the arts
These are so coherent with one another, and need each other by turns in their reasons, that if one is lacking the others cannot make a philosopher. Hence those seem to me to err who, not attending to such coherence in the arts, choose some of them for themselves and, leaving the others untouched, think they can become perfect by these.