The philosophy of Descartes was denounced as leading to irreligion; the doctrines of Locke were said to produce materialism; and in our own day, Dr. Buckland, Mr. Sedgwick, and other leading geologists, have been accused of overturning revelation with regard to the constitution and history of the earth. Indeed, there has scarcely been a discovery in astronomy, in natural history, or in physical science, that has not been attacked by the bigoted and narrow-minded as leading to infidelity. Other great discoverers, though they may not have been charged with irreligion, have had not less obloquy of a professional and public nature to encounter. When Dr. Harvey published his theory of the circulation of the blood, his practice fell off, [143] and the medical profession stigmatised him as a fool. "The few good things I have been able to do," said John Hunter, "have been accomplished with the greatest difficulty, and encountered the greatest opposition." Sir Charles Bell, while employed in his important investigations as to the nervous system, which issued in one of the greatest of physiological discoveries, wrote to a friend: "If I were not so poor, and had not so many vexations to encounter, how happy would I be!" But he himself observed that his practice sensibly fell off after the publication of each successive stage of his discovery. Thus, nearly every enlargement of the domain of knowledge, which has made us better acquainted with the heavens, with the earth, and with ourselves, has been established by the energy, the devotion, the self-sacrifice, and the courage of the great spirits of past times, who, however much they have been opposed or reviled by their contemporaries, now rank amongst those whom the enlightened of the human race most delight to honour. Nor is the unjust intolerance displayed towards men of science in the past, without its lesson for the present. It teaches us to be forbearant towards those who differ from us, provided they observe patiently, think honestly, and utter their convictions freely and truthfully. It was a remark of Plato, that "the world is God's epistle to mankind;" and to read and study that epistle, so as to elicit its true meaning, can have no other effect on a well-ordered mind than to lead to a deeper impression of His power, a clearer perception of His wisdom, and a more grateful sense of His goodness.