The four grounds on which Mr. Mill contends for the necessity of freedom in the expression of opinion to the mental wellbeing of mankind, are virtually contained in these.  His four grounds are, (1) that the silenced opinion may be true; (2) it may contain a portion of truth, essential to supplement the prevailing opinion; (3) vigorous contesting of opinions that are even wholly true, is the only way of preventing them from sinking to the level of uncomprehended prejudices; (4) without such contesting, the doctrine will lose its vital effect on character and conduct.

But Milton drew the line of liberty at what he calls ’neighbouring differences, or rather indifferences.’  The Arminian controversy had loosened the bonds with which the newly liberated churches of the Reformation, had made haste to bind themselves again, and weakened that authority of confessions, which had replaced the older but not more intolerant authority of the universal church.  Other controversies which raged during the first half of the seventeenth century,—­those between catholics and protestants, between prelatists and presbyterians, between socinians and trinitarians, between latitudinarians, puritans, and sacramentalists,—­all tended to weaken theological exclusiveness.  This slackening, however, was no more than partial.  Roger Williams, indeed, the Welsh founder of Rhode Island, preached, as early as 1631, the principles of an unlimited toleration, extending to catholics, Jews, and even infidels.  Milton stopped a long way short of this.  He did not mean ’tolerated popery and open superstition, which, as it extirpates all religious and civil supremacies, so itself should be extirpate, provided first that all charitable and compassionate means be used to win and regain the weak and the misled:  that also which is impious or evil absolutely either against faith or manners no law can possibly permit that intends not to unlaw itself.’

Locke, writing five-and-forty years later, somewhat widened these limitations.  His question was not merely whether there should be free expression of opinion, but whether there should furthermore be freedom of worship and of religious union.  He answered both questions affirmatively,—­not on the semi-sceptical ground of Jeremy Taylor, which is also one of the grounds taken by Mr. Mill, that we cannot be sure that our own opinion is the true one,—­but on the strength of his definition of the province of the civil magistrate.  Locke held that the magistrate’s whole jurisdiction reached only to civil concernments, and that ’all civil power, right, and dominion is bounded to that only care of promoting these things; and that it neither can nor ought in any manner to be extended to the saving of souls.  This chiefly because the power of the civil magistrate consists only in outward force, while true and saving religion consists in the inward persuasion of the mind, without which nothing can be acceptable to God, and such is the nature of the understanding