We have still several of his letters written from Louvestein, which acquaint us in what manner he spent his time.  He gave Vossius an account of his studies.  In the first of those Letters, without date, he observes to him that he had resumed the study of the Law, which had been long interrupted by his multiplicity of business; that the rest of his time he devoted to the study of Morality; which had led him to translate all the Maxims of the Poets collected by Stobaeus, and the fragments of Menander and Philemon.  He likewise purposed to extract from the Comic and Tragic Authors of Greece what related to Morality, and was omitted by Stobaeus, and to translate it into free verse, like that of the Latin Comic writers.  With regard to his translation of the fragments of the Greek Tragic authors, he intended that the verses of his Latin translation should resemble those of the original, excepting in the chorus’s, which he would put into the verse that best suited him.  He was in doubt whether he ought to print these additions with Stobaeus, and asks Vossius’s opinion whether he should place them at the end, or entirely new-mould that collection.  Sundays he employed in reading treatises on the truth of the Christian religion, and even spent some of his spare hours in this study:  on other days, when his ordinary labour was over, he meditated some work in Flemish on religion.  The subject which he liked best at that time was Christ’s love to mankind:  he no doubt intended to confute the extravagant opinions of the Gomarists.  He purposed also to write a Commentary on the Sermon on the mount.

Time seemed to pass very fast amidst these several projects.  December 15, 1619, he writes Vossius, that the Muses, which were always his delight, even when immersed in business, were now his consolation, and appeared more amiable than ever.  He wrote some short Notes on the New Testament:  these he intended to send to Erpenius, who was projecting a new edition of it; but a fit of illness obliged him to lay them aside[103].  When he was able to resume his studies, he composed in Dutch verse his treatise Of the Truth of the Christian Religion, and sent it to Vossius; who thought some places of it obscure.  It makes no mention of the Trinity or Incarnation, because, the authority and authenticity of the sacred Books once proved, these great points ought to be held demonstrated.  Those who since Grotius have written against infidelity with greatest success, have followed his example.  Sacred and profane authors employed him alternately.  In the end of the year 1620[104] he promises his brother to send him his observations on Seneca’s Tragedies:  These he had written at Vossius’s desire[105].  He acknowledges his conjectures are sometimes very bold; but is not so attached to them, but he will submit them to Vossius, and leaves them entirely to him.  We have seen that Du Maurier employed his best offices for Barnevelt and Grotius.  From the time they were arrested all correspondence between