Boerhaave had now for nine years read physical lectures, but without the title or dignity of a professor, when, by the death of professor Hotten, the professorship of physick and botany fell to him of course.

On this occasion he asserted the simplicity and facility of the science of physick, in opposition to those that think obscurity contributes to the dignity of learning, and that to be admired it is necessary not to be understood.

His profession of botany made it part of his duty to superintend the physical garden, which improved so much by the immense number of new plants which he procured, that it was enlarged to twice its original extent.

In 1714, he was deservedly advanced to the highest dignities of the university, and, in the same year, made physician of St. Augustin’s hospital in Leyden, into which the students are admitted twice a week, to learn the practice of physick.

This was of equal advantage to the sick and to the students, for the success of his practice was the best demonstration of the soundness of his principles.

When he laid down his office of governour of the university, in 1715, he made an oration upon the subject of “attaining to certainty in natural philosophy;” in which he declares, in the strongest terms, in favour of experimental knowledge; and reflects, with just severity, upon those arrogant philosophers, who are too easily disgusted with the slow methods of obtaining true notions by frequent experiments; and who, possessed with too high an opinion of their own abilities, rather choose to consult their own imaginations, than inquire into nature, and are better pleased with the charming amusement of forming hypotheses, than the toilsome drudgery of making observations.

The emptiness and uncertainty of all those systems, whether venerable for their antiquity, or agreeable for their novelty, he has evidently shown; and not only declared, but proved, that we are entirely ignorant of the principles of things, and that all the knowledge we have, is of such qualities alone as are discoverable by experience, or such as may be deduced from them by mathematical demonstration.

This discourse, filled as it was with piety, and a true sense of the greatness of the supreme being, and the incomprehensibility of his works, gave such offence to a professor of Franeker, who professed the utmost esteem for Des Cartes, and considered his principles as the bulwark of orthodoxy, that he appeared in vindication of his darling author, and spoke of the injury done him with the utmost vehemence, declaring little less than that the cartesian system and the Christian must inevitably stand and fall together; and that to say that we were ignorant of the principles of things, was not only to enlist among the skepticks, but to sink into atheism itself.

So far can prejudice darken the understanding, as to make it consider precarious systems as the chief support of sacred and invariable truth.