Great men singers are born with so complete a co-ordination of the laryngeal muscles that, as a rule, they do not know that there are two registers, and some of them would be most indignant if they were told that they were using falsetto. Unfortunately for the teaching of this subject those who sing best generally know the least about vocal technic, since the apparatus works perfectly and unconsciously for them. When great singers commit themselves to paper they do indeed become quite delightfully childish on this subject. But what does a great singer know of acoustics or physiology? It is not to be expected of him.
Stanley, Douglas, and Stanley Sylvester A. Watkins. The Science of Voice... C. Fischer, 1948.