It is very much the custom, in modern writing about liberty of thought, to pass lightly over the central difficulty, which sooner or later will have to be considered. The difficulty is this, that the freedom of the intellectual life can never be secured except by treating as if they were doubtful several affirmations which large masses of mankind hold to be certainties as indisputable as the facts of science. One of the most recently conspicuous of these affirmations is the infallibility of the Pope of Rome. Nothing can be more certain in the opinion of immense numbers of Roman Catholics than the infallible authority of the Supreme Pontiff on all matters affecting doctrine. But then the matters affecting doctrine include many subjects which come within the circle of the sciences. History is one of those subjects which modern intellectual criticism takes leave to study after its own methods, and yet certain prevalent views of history are offensive to the Pope and explicitly condemned by him. The consequence is, that in order to study history with mental liberty, we have to act practically as if there existed a doubt of the Papal infallibility. The same difficulty occurs with reference to the great Protestant doctrine which attributes a similar infallibility to the various authors who composed what are now known to us as the Holy Scriptures. Our men of science act, and the laws of scientific investigation compel them to act, as if it were not quite certain that the views of scientific subjects held by those early writers were so final as to render modern investigation superfluous. It is useless to disguise the fact that there is a real opposition of method between intellect and faith, and that the independence of the intellectual life can never be fully secured unless all affirmations based upon authority are treated as if they were doubtful. This implies no change of manner in the intellectual classes towards those classes whose mental habits are founded upon obedience. I mean that the man of science does not treat the affirmations of any priesthood with less respect than the affirmations of his own scientific brethren; he applies with perfect impartiality the same criticism to all affirmations, from whatever source they emanate. The intellect does not recognize authority in any one, and intellectual men do not treat the Pope, or the author of Genesis, with less consideration than those famous persons who in their day have been the brightest luminaries of science. The difficulty, however, remains, that whilst the intellectual class has no wish to offend either those who believe in the infallibility of the Pope, or those who believe in the infallibility of the author of Genesis, it is compelled to conduct its own investigations as if those infallibilities were matters of doubt and not of certainty.