Jack Spratt on the First Law of Dietetics

The good Doctor on dietetics:

Now you will often come across people who advocate ways of living or methods of treating diseases which completely ignore that [no two of us are alike], and which are therefore obviously wrong. I have often said that the first fundamental law of dietetics is just that old word which tells us that:   

          Jack Spratt could eat no fat,
          his wife could eat no lean.

Quite right! It is amusing in one sense but on the other hand it is a very vital, fundamental principle for dietetics. Constitutionally Jack Spratt and his wife are different, and to suggest that the same diet would be the best for both persons is to be guilty of a fundamental fallacy.  

—D. Martyn Lloyd-Jones, Spiritual Depression (Hannibal: Granted Ministries), 9.