The temperate tone of the remarks made by Albertus on the medical virtues of plants contrasts favourably with the puerilities of many later writers. Much of the criticism from which he has suffered at various times has been, in reality, directed against a book called ‘De virtutibus herbarum,’ the authorship of which was quite erroneously attributed to him. We shall refer to this work again in Chapter VIII. After the time of Albertus, no great student of Aristotelian botany arose before Andrea Cesalpino, whose writings, which belong to the end of the sixteenth century, will be considered in a later chapter. The work of Cesalpino had great qualities, but, curiously enough, it had little influence on the science of his time. He may be regarded as perhaps the last important representative of Aristotelian botany. 3. MEDICINAL BOTANY. With the Revival of Learning, the speculative botany of the ancients began to lose its hold upon thinking men. This may be attributed to the curious lack of vitality, and the absence of the power of active development, manifested in this aspect of the subject since its initiation at the hands of Aristotle. It had proved comparatively barren, because, though the minds which engaged in it were among the finest that have ever been concerned with the science, the basis of observed fact was inadequate in quality and quantity to sustain the philosophical superstructure built upon it. It might have been supposed _a priori_ that accurate observation of natural phenomena needed a less highly evolved type of mind than that required to cope with metaphysical considerations, and hence that, in the development of any science, the epoch of observation would have preceded the epoch of speculation. In actual fact, however, the reverse appears to have been the case. The power of scientific observation seems to have lagged many centuries behind the power of reasoning, and to have reached its maturity at least two thousand years later. Aristotle and Theophrastus arrived by the subtlest mental processes at a certain attitude towards the universe, and at certain ideas concerning the nature of things. They attempted a direct advance in scientific thought by extending these conceptions to include the plant world. It was an heroic effort, but one which could not ultimately form a basis for continued progress, because, in its inception, preconceived ideas had come first, and the facts of Nature second. It seems to be almost a law of thought, that it is the indirect advances which in the end prove to be the most fertile. The progress of a science, like that of a sailing boat, more often proceeds by means of “tacking” than by following a direct course.