In 1608 he went to Paris, enjoying the friendship and hospitality of the old Constable de Montmorency and meeting King Henry IV. On his return, as he says himself, he was "in great esteem both in court and city, many of the greatest desiring my company." In 1610 he served as a volunteer in the Low Countries under the Prince of Orange, whose intimate friend he became, and distinguished himself at the capture of Juliers from the emperor. He offered to decide the war by engaging in single combat with a champion chosen from among the enemy, but his challenge was declined. During an interval in the fighting he paid a visit to Spinola, in the Spanish camp near Wezel, and afterwards to the elector palatine at Heidelberg, subsequently travelling in Italy. At the instance of the Duke of Savoy he led an expedition of 4,000 Huguenots from Languedoc into Piedmont to help the Savoyards against Spain, but after nearly losing his life in the journey to Lyon he was imprisoned on his arrival there, and the enterprise came to nothing. Thence he returned to the Netherlands and the Prince of Orange, arriving in England in 1617.
In 1619, Herbert was made ambassador to Paris, but a quarrel with de Luynes and a challenge sent by him to the latter occasioned his recall in 1621. After the death of de Luynes, Herbert resumed his post in February 1622. He was very popular at the French court and showed considerable diplomatic ability, his chief objects being to accomplish the marriage between Charles and Henrietta Maria and secure the assistance of Louis XIII for the elector palatine. He failed in the latter, and was dismissed in April 1624. He returned home greatly in debt and received little reward for his services beyond the Irish peerage of Castle Island on 31 May 1624 and the English barony of Cherbury, or Chirbury, on May 7, 1629.
In 1632 he was appointed a member of the council of war. He attended the king at York in 1639, and in May 1642 was imprisoned by the parliament for urging the addition of the words "without cause" to the resolution that the king violated his oath by making war on parliament. He determined after this to take no further part in the struggle, retired to Montgomery Castle, and declined the king's summons. On September 5, 1644 he surrendered the castle to the parliamentary forces, returned to London, submitted, and was granted a pension of £20 a week. In 1647. he paid a visit to Pierre Gassendi at Paris, and died in London the following summer, being buried in the church of St Giles's in the Fields.
Lord Herbert left two sons, Richard (c. 1600-1655), who succeeded him as 2nd Lord Herbert of Cherbury, and Edward, the title becoming extinct in the person of Henry Herbert, the 4th baron, grandson of the 1st Lord Herbert, in 1691. In 1694, however, it was revived in favour of another Henry Herbert (1654-1709), son of Sir Henry Herbert (1595-1673), brother of the 1st Lord Herbert of Cherbury. Lord Herbert's cousin and namesake, Sir Edward Herbert, was also a prominent figure in the English Civil War.
Herbert's first and most important work is the De veritate, prout distinguitur a revelatione, a verisimili, a possibili, et a falso (On Truth, as it is Distinguished from Revelation, the Probable, the Possible, and the False) (Paris, 1624; London, 1633; translated into French in 1639 and into English in 1937). It combines a theory of knowledge with a partial psychology, a methodology for the investigation of truth, and a scheme of natural religion. The author's method is prolix and often far from clear; the book is no compact system, but it contains the skeleton and much of the soul of a complete philosophy. Giving up all past theories as useless, Herbert professedly endeavours to constitute a new and true system. Truth, which he defines as a just conformation of the faculties with one another and with their objects, he distributed into four classes or stages:
The faculties of the mind are as numerous as the differences of their objects, and are accordingly innumerable; but they may be arranged in four groups. The first and fundamental and most certain group is the Natural Instinct, to which belong the notitiae communes, which are innate, of divine origin and indisputable. The second group, the next in certainty, is the sensus internus (under which head Herbert discusses amongst others love, hate, fear, conscience with its communis notitia, and free will); the third is the sensus externus; and the fourth is discursus, reasoning, to which, as being the least certain, we have recourse when the other faculties fail. The ratiocinative faculties proceed by division and analysis, by questioning, and are slow and gradual in their movement; they take aid from the other faculties, those of the instinctus naluralis being always the final test. Herbert's categories or questions to be used in investigation are ten in number whether (a thing is), what, of what sort, how much, in what relation, how, when, where, whence, wherefore. No faculty, rightly used, can err "even in dreams"; badly exercised, reasoning becomes the source of almost all our errors. The discussion of the notitiae communes is the most characteristic part of the book.
The exposition of them, though highly dogmatic, is at times strikingly Kantian in substance. "So far are these elements or sacred principles from being derived from experience or observation that without some of them, or at least some one of them, we can neither experience nor even observe." Unless we felt driven by them to explore the nature of things, "it would never occur to us to distinguish one thing from another."
It cannot be said that Herbert proves the existence of the common notions; he does not deduce them or even give any list of them. But each faculty has its common notion; and they may be distinguished by six marks, their priority, independence, universality, certainty, necessity (for the well-being of man), and immediacy. Law is based on certain common notions; so is religion. Though Herbert expressly defines the scope of his book as dealing with the intellect, not faith, it is the common notions of religion he has illustrated most fully; and it is plain that it is in this part of his system that he is chiefly interested. The common notions of religion are the famous five articles, which became the charter of the English debits.
His achievements in historical writing are vastly inferior, and vitiated by personal aims and his preoccupation to gain the royal favour. Herbert's first historical work is the Expeditio Buckinghami ducis (published in a Latin translation in 1656 and in the original English by the Earl of Powis for the Philobiblon Society in 1860), a defence of Buckingham's conduct of the ill-fated expedition of 1627. The Life and Raigne of King Henry VIII (1649) derives its chief value from its composition from original documents, but is ill-proportioned, and the author judges the character and statesmanship of Henry with too obvious a partiality.
His poems, published in 1665 (reprinted and edited by John Churton Collins in 1881), show him in general a faithful disciple of Donne, obscure and uncouth. His satires are miserable compositions, but a few of his lyrical verses show power of reflection and true inspiration, while his use of the metre afterwards employed by Tennyson in his "In Memoriam" is particularly happy and effective. His Latin poems are evidence of his scholarship. Three of these had appeared together with the De causis errorum in 1645. To these works must be added A Dialogue between a Tutor and a Pupil (1768; a treatise on education, manuscript in the Bodleian Library); a treatise on the king's supremacy in the Church (manuscript in the Record Office and at the Queen's College, Oxford), and his well-known autobiography, first published by Horace Walpole in 1764, a naïve and amusing narrative, too much occupied, however, with his duels and amorous adventures, to the exclusion of more creditable, incidents in his career, such as his contributions to philosophy and history, his intimacy with Donne, Ben Jonson, Selden and Carew, Casaubon, Gassendi and Grotius, or his embassy in France, in relation to which he only described the splendour of his retinue and his social triumphs.