The two primary necessities are, of course, voice and ear. Without voice a singer would be like a painter without paints. Without ear he would be in a parlous a state as a painter without eyes.
The voice must be there, for, no matter what may be said to the contrary, no teacher can bring a voice into existence. He can show a pupil how to use the voice properly, and he can improve it by means of various exercises, he can instruct how it may be shown off to the best advantage, but he cannot create a voice. He is like a diamond-cutter, who given a rough diamond, can polish and cut it till it shines with all the brilliancy that lay hidden under its rough surface, but cannot take a piece of clay and polish and cut that till it shines with the dazzling lustre of the diamond.
The ear is doubly necessary, first for regulating pitch and this enabling any one to sing in tune; secondly, for hearing and reproducing the various timbres of the voice. The habit of listening critically to one's own voice and to the voices of others is of the utmost importance.
—MacKinlay, Sterling Malcom. The Singing Voice and Its Training (1910): 9-10. MacKinlay was one of the last four-year students of Manuel García, his mother, Antoinette Sterling, having studied with the great master.