In the ordinary branches of human knowledge or inquiry, the judicious questioning of received opinions has been the sign of scientific vitality, the principle of scientific advancement, the very source and root of healthy progress and growth. If medicine had been regulated three hundred years ago by Act of Parliament; if there had been Thirty-nine Articles of Physic, and every licensed practitioner had been compelled, under pains and penalties, to compound his drugs by the prescriptions of Henry the Eighth's physician, Doctor Butts, it is easy to conjecture in what state of health the people of this country would at present be found. Constitutions have changed with habits of life, and the treatment of disorders has changed to meet the new conditions. New diseases have shown themselves of which Doctor Butts had no cognizance; new continents have given us plants with medicinal virtues previously unknown; new sciences, and even the mere increase of recorded experience, have added a thousand remedies to those known to the age of the Tudors. If the College of Physicians had been organized into a board of orthodoxy. and every novelty of treatment had been regarded as a crime against society, which a law had been established to punish, the hundreds who die annually from preventible causes would have been thousands and tens of thousands. Astronomy is the most perfect of the sciences. The accuracy of the present theory of the planetary move merits is tested daily and hourly by the most delicate experiments, and the legislature, if it so pleased, might enact the first principles of these movements into a statute, without danger of committing the law of England to falsehood. Yet, if the legislature were to venture on any such paternal procedure, in a few years gravitation itself would be called in question, and the whole science would wither under the fatal shadow. There are many phenomena still unexplained to give plausibility to scepticism; there are others more easily formularized for working purposes in the language of Ptolemy; and there would be reactionists who would invite us to return to the safe convictions of our forefathers. What the world has seen the world may see again; and were it once granted that astronomy were something to be ruled by authority, new Popes would imprison new Galileos; the knowledge already acquired would be strangled in the cords which were intended to keep it safe from harm, and deprived of the free air on which its life depends it would dwindle and die.