In 1628 Gassendi visited Holland with a view to cultivate an acquaintance with the philosophers of that country. During his residence there he composed, at the instance of his friend 3Iereenne, the work entitled Examen philosophicum Fludd,' in answer to the dissertation of our countryman on the subject of the Mosaic philosophy. Upon his return to Digne, Gassendi applied himself with great diligence to astronomical studies, for which his fondness had grown with his years, and he had the good fortune, on the 7th of November 1631, to be the first to observe a transit of the planet Mercury over the sun's disc which had been previously calculated by Kepler.
In the year 1641, being called to Paris by a law-suit arising out of the affairs of the chapter, his amiable disposition and brilliant talents obtained for Gassendi the regard and esteem of the most distinguished persons of the metropolis of France, and the friendship of the Cardinal Richelieu and of his brother the Cardinal du Plessis, archbishop of Lyon. At this period Des Cartes, with whom Gassendi had long maintained a close and friendly intercourse, was working a reform in philosophy, and by the publication of his Meditationes ' had opened for it a new and more useful career. In this work however Gassendi discovered much that was objectionable, and forthwith attacked the philosophical system of his friend in a work entitled Disquisitio Mctaphysica, seu Dubitationes ad Meditationes Cartesii,' which was put into the hands of Des Cartes by their mutual friend Mersenne. Des Cartes wrote an answer, which he published together with the 'Doubts,' under the head, 'Sixth Objection to the Meditations.' In 1643 Gassendi composed the Instantias' in reply, and circulated them in manuscript in Paris before he sent them to M. Sorbiere to be printed at Amsterdam. The latter circumstance tended to confirm sod widen the difference which, in the course of the controversy, had grown up between the two friends, who however entertained a sincere respect for each other, and were eventually reconciled by the kindly offices of a common friend, the Abb6 d'Estrees. Bdllet, the grapher of Des Cartes, ascribes the publication of the 'Doubts' to secret jealousy of the growing fame of the author of the 'Medita tions,' and to chagrin on the part of Gassendi at the omission iu Des Cartes 's Treatise of Meteors of his Dissertation upou the singular phenomenon of two parhelia which had been observed at Rome. But the mind of Gassendi seems to have been superior to the influence of such paltry motives, and the origin of the work in question may more justly be referred to the love of truth, which to Gasseudi was dearer than friendship itself. Moreover, there was much in their respective characters that was calculated to lead to difference of opinion upon speculative matters. Carried away by a lively imagination, Des Cartes
thought it sufficient to draw from his own mind and his individual consciousness the materials for constructing a new system of philo sophy; whereas Gassendi, a man of immense learning, and the declared enemy of whatever had the appearance of novelty, was strongly biassed in favour of antiquity. Chimaera for chimaera, he preferred that which had at leant the prescription of 2000 years in its favour. From Democritus and Epicurus, whose opinions were above all others most easily recoucileable with his own scientific information, Gassendi drew whatever was well-founded and ratioual in their system to form the basis of his own physiology. Having restored the doctrine of Atoms and a Void with such slight modifica tion, that at most perhaps he did but lend to it a modern style and language, his philosophy had the glory of dividing with Des Cartes the empire of the French philosophical world.
In 1645 Gassendi was appointed professor of mathematics in the College Royal of Paris, upon the nomination and by the influence of Cardinal du Plessis. As this institution was intended principally for the advancement of astronomy, he read lectures upon that science to a crowded sod distinguished audience, by which he increased the reputation he had previously acquired, and quickly became the focus of the literary activity of France, so far as it was directed to his favourite sciences of mathematics and astronomy.
But the intensity of his studies had undermined the constitution of Gassondi, and a severe cold having occasioned inflammation of the lungs, he was forced to retire to Digne for the restoration of his health. In this retirement however he was far from idle. In 1647 he published his principal work, 'De Vitl et Moribus Epicuri,' in which he clears the character of this philosopher from the mist of prejudice with which it had been invested and unfairly handed down to posterity. The 'Syntagma Philosophies Epicurean; which followed in 1649, is an attempt to reconstruct the system of Epicurus out of the extant fragments, and to give a complete and connected exposition of his theory. Notwithstanding the express refutation, which Gassendi subjoined, of the errors, both physical and moral, of this philosopher, and despito the purity of his own moral character and the exactitude of his religious observances, the sincerity of his religious belief was doubted by those who were constrained to admit the learning and critical acuteness which the work displayed ; eventually however the injustice of the calumny redounded to the disgrace of his envious traducers.