The principles that guided the founders of the CIE 1931 system for colorimetry are examined. The principles are applied to the Wright-Guild experimental determinations of the color mixture data to show in detail how and why each step in the development of the CIE 1931 system for colorimetry came about. These steps are examined in the light of 65 years advanced knowledge of colorimetry. The necessity for each of these principles in the model world is examined critically to determine whether one might hold to the same principles if the system were being freshly formulated today.