post hoc rationalization

The courts may not accept appellate counsel's post hoc rationalizations for agency action; Chenery requires that an agency's discretionary order be upheld, if at all, on the same basis articulated in the order by the agency itself: '(A) simple but fundamental rule of administrative law * * * is * * * that a reviewing court, in dealing with a determination or judgment which an administrative agency alone is authorized to make, must judge the propriety of such action solely by the grounds invoked by the agency. If those grounds are inadequate or improper, the court is powerless to affirm the administrative action * * *.