“Prudence is not only the first in rank of the virtues political and moral, but she is the director, the regulator, the standard of them all. Metaphysics cannot live without definition; but prudence is cautious how she defines. Our courts cannot be more fearful in suffering fictitious cases to be brought before them for eliciting their determination on a point of law, than prudent moralists are in putting extreme and hazardous cases of conscience upon emergencies not existing.”