A Frame-Free Formulation of Elasticity
The term “principle of material frame-indifference” was introduced in 1965 by C. Truesdell and me in our contribution [NLFT] to the Encyclopedia of Physics. Earlier, I had used the term “principle of objectivity”, and some people use this term to this day. I meant “objectivity” to express independence of the “observer”, but Truesdell disliked the term as being too easily misinterpreted. In fact, I now believe that the principle has nothing to do with “observer”, who is defined in the dictionary to be a person. What matters is the use of frames of reference as a means to make precise the concept of location, as explained in Sect.4 of [N2] of [FC].