The Post constraint for linear electromagnetic materials arises from two basic tenets of modern electromagnetism. The first is the microscopic basis of all electromagnetic phenomenons. The second is that E(x,t) and B(x,t) are the fundamental fields, whereas D(x,t) and H(x,t) are merely secondary constructs.
The latter is in contrast to the older version of electromagnetism, wherein the roles of B(x,t) and H(x,t) are the opposite, and which does not admit the Post constraint. A comprehensive view of the Post constraint is presented.