In Stanley v. Illinois (1972) 405 U.S. 645 [92 S. Ct. 1208, 31 L. Ed. 2d 551] the United States Supreme Court injected federal constitutional law into the state law of paternity by establishing the idea that it was subject to limits under the due process clause of the Fourteenth Amendment to the Constitution. That is, the operation of certain substantive rules of state law might not be "constitutionally defensible." (See id. at p. 652 [92 S. Ct. at p. 1213].) In Stanley, a state statute automatically made the children of unwed fathers the wards of the court upon the death of the mother. Thus in Stanley, a man who had lived with the mother of his children for 18 years, but never married her, was separated from his children at the mother's death. In the opening lines of the opinion the Stanley court captured the mechanistic ruthlessness with which the Illinois statute operated: "When J Stanley died, P Stanley lost not only her but also his children." ( Id. at p. 646 [92 S. Ct. at p. 1210].) The Supreme Court would have none of it. It held that the due process clause "mandated" that P Stanley be given a hearing to show his parental fitness. (405 U.S. at p. 657 [92 S. Ct. at p. 1216].)