Grotius had a brother named William, with whom he kept up the greatest intimacy during his whole life, and made him the confident of his studies and designs.  It appears by his letters that they lived in the strictest friendship.  Hugo, who was the eldest, contributed to his brother’s education, and directed his studies.  We have a letter from Grotius to his brother, dated at Rotterdam September 28, 1614, containing a plan of study.  “I am of opinion, says he, that in order to acquire the knowledge of Law, before you touch upon law tracts you should read with attention Aristotle’s second and fifth book of Ethics, to Nicomachus, or the excellent paraphrase of them published by Heinsius; then Aristotle’s Rhetoric, with the learned commentary on it; afterwards Cicero’s Offices, the Paradoxes, De Finibus, Of Laws, the Topica, and De Inventiene.  I could wish that whilst reading you would make extracts, or at least mark in the margin of your book whatever has relation to the Law of Nature and of Nations, to the origin of Laws and Magistrates, to Jus publicum et privatum.  When you have done this, we shall think of the rest.”  It was Grotius who corrected his brother’s Law Theses.

William Grotius came to France in 1617 to learn the language; and retiring to Senlis made great progress in it:  he purposed to go to Tours, and Grotius approved of his journey, because the air was pure, and they spoke good French there.

William Grotius, as well as his brother, had a turn for theological studies:  he wrote something in verse on the Decalogue, which Grotius mentions in a letter dated from his prison at Louvestein, November 1, 1620.  “I have read with pleasure, he says, what you have done on the Decalogue:  the maxims are excellent, and the verses easy.”

William had his brother’s confidence during his whole life.  Grotius writes to him from Paris, April 14, 1622, “You are never weary of assisting me under my afflictions:  if ever fortune enables me to testify my gratitude, I will forgive her all the tricks she has played me.”  He was desirous, in the end of the year 1622, that his brother should settle his matters so, as to come to see him in the beginning of the following year; but this journey did not take place.  Grotius’s disgrace affected his brother:  he despaired of attaining to honours, and Grotius advised him to think only of raising himself by the study of the Law.

In April 1623 he married Alida Grasvinkel.  About this time a Dutchman was seized at Lillo, with letters from William Grotius to his brother.  It was expected that something would be found in them against the State, and they talked of nothing less than imprisoning him; but notwithstanding the malice of his enemies, they could not find the least pretext from these letters to trouble him.  In the mean time William followed the profession of an Advocate with much success:  Grotius compliments him on it in a letter of the 28th of November, 1625, in which he tells him, that the life he led in shining at the bar was much more agreeable than that which is spent in public employments.