"In the theory of plasticity, stress is related to strain through the Prandtl-Reuss equations, and the stress is a function of the displacement field."
It is very rudimental and standard statement of plasticity theory. This kind of statement can be seen at any textbook. I have seen this kind of introductory statements in many places of many publications.
Is it okay to state like this? "In the theory of plasticity, stress and strain is related through the Prandtl-Reuss equations, and the stress depends on the displacement field." Heck, what is the difference?
These guys are fussing over nothing.
I repeat, this statment is just like saying:
In the theory of mathematics, positive number is related to negative number through '-' operator, and the positive number is a function of nonsense.