Whether a particular act is, in fact, illegal may have to await final determination by a court of law or other adjudicative body.
Whether a particular act is illegal depends purely on the opinion of the doer and not on any other governing court. 
mnli hypothesis: Whether a particular act is illegal depends purely on the opinion of the doer and not on any other governing court.  premise: Whether a particular act is, in fact, illegal may have to await final determination by a court of law or other adjudicative body.