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. This treatment of Boerhaave was so far resented by the governours of his university, that they procured from Franeker a recantation of the invective that had been thrown out against him: this was not only complied with, but offers were made him of more ample satisfaction; to which he returned an answer not less to his honour than the victory he gained, "that he should think himself sufficiently compensated, if his adversary received no further molestation on his account." So far was this weak and injudicious attack from shaking a reputation not casually raised by fashion or caprice, but founded upon solid merit, that the same year his correspondence was desired upon botany and natural philosophy by the academy of sciences at Paris, of which he was, upon the death of count Marsigli, in the year 1728, elected a member. Nor were the French the only nation by which this great man was courted and distinguished; for, two years after, he was elected fellow of our Royal society.