Many of these pages are devoted to the statement of Quaker theology; some are occupied with descriptions of his colonial possessions; some are given to counsels and conclusions drawn from experience and dealing with human life in general; but there is one idea which continually recurs,—sometimes made the subject of a thesis, sometimes entering by the way,—and that is the popular right of liberty of conscience. It was for this that he worked, and chiefly lived, most of his life. Here it is set forth with all clearness in the first public word which he wrote.

William's letter opened the jail doors. It is likely, however, that the signature was more influential than the epistle; for his Quaker associates seem not to have come out with him. The fact which probably weighed most with the Lord President was that Penn was the son of his father the admiral, and the protégé of Ormond. His father called him home. It was on the 3d of September that William was arrested; on the 29th of December, being the Lord's day, Mrs. Turner calls upon Mr. and Mrs. Pepys for an evening of cheerful conversation, "and there, among other talk, she tells me that Mr. William Pen, who has lately come over from Ireland, is a Quaker again, or some very melancholy thing; that he cares for no company, nor comes into any."

Admiral Penn was sorely disappointed. Neither France nor Ireland had availed to wean his son from his religious eccentricities. Into the pleasant society where his father had hoped to see him shine, he declined to enter. He said "thee" and "thou," and wore his hat. Especially upon these points of manners, the young man and his father held long discussions. The admiral insisted that William should refrain from making himself socially ridiculous; though even here he was willing to make a reasonable compromise. "You may 'thee' and 'thou' whom you please," he said, "except the king, the Duke of York, and myself." But the young convert declined to make any exceptions.

Thereupon, for the second time, the admiral thrust his son out of the house. The Quakers received him. He was thenceforth accounted among them as a teacher, a leader: in their phrase, a "public Friend." This was in 1668, when he was twenty-four years old.

The work of a Quaker minister, at that time, was made interesting and difficult not only by the social and ecclesiastical prejudices against which he must go, but by certain laws which limited free speech and free action. The young preacher speedily made himself obnoxious to both these kinds of laws. Of the three years which followed, he spent more than a third of the time in prison, being once confined for saying, and twice for doing, what the laws forbade.

The religious world was filled with controversy. There were discussions in the meeting-houses; and a constant stream of pamphlets came from the press, part argument and part abuse. Even mild-mannered men called each other names. The Quakers found it necessary to join in this rough give-and-take, and Penn entered at once into this vigorous exercise. He began a long series of like documents with a tract entitled "Truth Exalted." The intent of it was to show that Roman Catholics, Churchmen, and Puritans alike were all shamefully in error, wandering in the blackness of darkness, given over to idle superstition, and being of a character to correspond with their fond beliefs; meanwhile, the Quakers were the only people then resident in Christendom whose creed was absolutely true and their lives consistent with it.

"Come," he says, "answer me first, you Papists, where did the Scriptures enjoin baby-baptism, churching of women, marrying by priests, holy water to frighten the devil? Come now, you that are called Protestants, and first those who are called Episcopalians, where do the Scriptures own such persecutors, false prophets, tithemongers, deniers of revelations, opposers of perfection, men-pleasers, time-servers, unprofitable teachers?" The Separatists are similarly cudgeled: they are "groveling in beggarly elements, imitations, and shadows of heavenly things."

Presently, a Presbyterian minister named Vincent attacked Quakerism. Joseph Besse, Penn's earliest biographer, says that Vincent was "transported with fiery zeal;" which, as he remarks in parenthesis, is "a thing fertile of ill language." Penn challenged him to a public debate; and, this not giving the Quaker champion an opportunity to say all that was in his mind, he wrote a pamphlet, called "The Sandy Foundation Shaken." The full title was much longer than this, in the manner of the time, and announced the author's purpose to refute three "generally believed and applauded doctrines: first, of one God, subsisting in three distinct and separate persons; second, of the impossibility of divine pardon without the making of a complete satisfaction; and third, of the justification of impure persons by an imputed righteousness."

Penn's handling of the doctrine of the Trinity in this treatise gave much offense. He had taken the position of his fellow-religionists, that the learning of the schools was a hindrance to religion. He sought to divest the great statements of the creed from the subtleties of mediæval philosophy. He purposed to return to the Scripture itself, back of all councils and formulas. Asserting, accordingly, the being and unity of Father, Son, and Holy Ghost, he so refused all the conventional phrases of the theologians as to seem to them to reject the doctrine of the Trinity itself. He did deny "the trinity of distinct and separate persons in the unity of essence." If the word "person" has one meaning, Penn was right; if it has another meaning, he was wrong. If a "person" is an individual, then the assertion is that there are three Gods; but if the word signifies a distinction in the divine nature, then the unity of God remains. As so often happens in doctrinal contention, he and his critics used the same words with different definitions. The consequence was that the bishop of London had him put in prison. He was restrained for seven months in the Tower.

The English prison of the seventeenth century was a place of disease of body and misery of mind. Penn was kept in close confinement, and the bishop sent him word that he must either recant or die a prisoner. "I told him," says Penn, "that the Tower was the worst argument in the world to convince me; for whoever was in the wrong, those who used force for religion could never be in the right." He declared that his prison should be his grave before he would budge a jot. Thus six months passed.

But the situation was intolerable. It is sometimes necessary to die for a difference of opinion, but it is not advisable to do so for a simple misunderstanding. Penn and the bishop were actually in accord. The young author therefore wrote an explanation of his book, entitled "Innocency with her Open Face." At the same time he addressed a letter to Lord Arlington, principal secretary of state. In the letter he maintained that he had "subverted no faith, obedience or good life," and he insisted on the natural right of liberty of conscience: "To conceit," he said, "that men must form their faith of things proper to another world by the prescriptions of mortal men, or else they can have no right to eat, drink, sleep, walk, trade, or be at liberty and live in this, to me seems both ridiculous and dangerous." These writings gained him his liberty. The Duke of York made intercession for him with the king.

Penn had occupied himself while in prison with the composition of a considerable work, called "No Cross, No Crown." It is partly controversial, setting forth the reasons for the Quaker faith and practice, and