The acquisition of sound knowledge in astronomy and likewise in every other science rests primarily upon the observation of natural facts or phenomena and then upon deducing rational conclusions from such observation. Yet this seemingly simple rule had not been continuously and effectively applied in any period of history prior to the sixteenth century. The scientific method of most of the medieval as well as of the ancient scholars was essentially that of Aristotle. [Footnote: Exception to this sweeping generalization must be made in favor of several medieval scientists and philosophers, including--Roger Bacon, a Franciscan friar of the thirteenth century.] This so-called deductive method of Aristotle assumed as a starting-point some general of principle as a premise or hypothesis and thence proceeded, by logical reasoning, to deduce concrete applications or consequences. It had been extremely valuable in stimulating the logical faculties and in showing men how to draw accurate conclusions, but it had shown a woeful inability to devise new general principles. It evolved an elaborate theology and a remarkable philosophy, but natural experimental science progressed relatively little until the deductive method of Aristotle was supplemented by the inductive method of Francis Bacon. [Sidenote: Modern Method of Science: Introduction. Francis Bacon] Aristotle was partially discredited by radical humanists, who made fun of the medieval scholars who had taken him most seriously, and by the Protestant reformers, who assailed the Catholic theology which had been carefully constructed by Aristotelian deduction. But it was reserved for Francis Bacon, known as Lord Bacon (1561-1626), to point out all the shortcomings of the ancient method and to propose a practicable supplement. A famous lawyer, lord chancellor of England under James I, a born scientist, a brilliant essayist, he wrote several philosophical works of first-rate importance, of which the _Advancement of Learning_ (1604) and the _Novum Organum_ (1620) are the most famous. It is in these works that he summed up the faults which the widening of knowledge in his own day was disclosing in ancient and medieval thought and set forth the necessity of slow laborious observation of facts as antecedent to the assumption of any general principle. [Sidenote: Descartes]