Purpose was to fuse into one nation the independent sovereign states; therefore, P&Is only apply to fundamental rights
b.    fundamental:  for the promotion of interstate harmony; rights that are important to the maintenance & well-being of the union
c.    from Slaughter-house:  P&Is are those rights which are “fundamental, which belong of right to the citizens of all free government, and which have at all times been enjoyed by citizens of the several states which compose this union, from the time of their becoming free, independent and sovereign.”