To Daniel Heinsius he writes, April 13, 1613, "I am very well; and cannot say enough of my felicity in enjoying the friendship of such a great man as Grotius.

O that incomparable man! I knew him before: but fully to comprehend the excellency of his divine genius, one must see him, and hear him converse.

His countenance speaks probity, and his discourse discovers the deepest learning and the most sincere piety.
Think not that I only am his admirer; all learned and good men entertain the same sentiments for him, particularly the King." Casaubon writes to the President de Thou, April 20, 1613, to acquaint him what pleasure he had received by seeing Grotius.

"I must let you know, says he, that I have seen here Hugo Grotius.

He is a man of admirable probity and learning[63]." They had long conversations together on religion.