The unity of knowledge was thus purchased at a price. Things must cease to be studied in themselves, and must be allegorized into types, in order that they might be reduced to a unity. Perhaps the purchase of unity on terms such as these is a bad bargain; and it is at any rate obvious that in such an atmosphere scientific thought will not flourish, or man learn to adjust himself readily to the laws of his environment. From the standpoint of natural science we may readily condemn the Middle Ages and all their works; and we may prefer a single _Opus_ of Roger Bacon to the whole of the _Summa_ of St. Thomas. But it is necessary to judge an age which was destitute of natural science by some other criterion than that of science; nor must we hasten to say that the Middle Ages found the Universal so easily, because they ignored the Particular so absolutely. The truth is, that though mediaeval thinkers knew far more of the writings of Aristotle than they did of those of Plato, they were none the less far better Platonists than they were Aristotelians. If they had been better Aristotelians, they would have been better biologists; but as they were good Platonists, they had a conception of the purpose and system of human life in society, which perhaps excuses all, and more than all, the defects of their biology. Any survey, however brief, of the political theory of the Middle Ages will show at once its Platonic character and its incessant impulse towards the achievement of unity. III