Mr. Mill's memorable plea for social liberty was little more than an enlargement, though a very important enlargement, of the principles of the still more famous Speech for Liberty of Unlicensed Printing with which Milton ennobled English literature two centuries before. Milton contended for free publication of opinion mainly on these grounds: First, that the opposite system implied the 'grace of infallibility and incorruptibleness' in the licensers. Second, that the prohibition of bold books led to mental indolence and stagnant formalism both in teachers and congregations, producing the 'laziness of a licensing church.' Third, that it 'hinders and retards the importation of our richest merchandise, truth;' for the commission of the licenser enjoins him to let nothing pass which is not vulgarly received already, and 'if it come to prohibiting, there is not aught more likely to be prohibited than truth itself, whose first appearance to our eyes, bleared and dimmed with prejudice and custom, is more unsightly and unplausible than many errors, even as the person is of many a great man slight and contemptible to see to.' Fourth, that freedom is in itself an ingredient of true virtue, and 'they are not skilful considerers of human things who imagine to remove sin by removing the matter of sin; that virtue therefore, which is but a youngling in the contemplation of evil, and knows not the utmost that vice promises to her followers, and rejects it, is but a blank virtue, not a pure; her virtue is but an excremental virtue, which was the reason why our sage and serious poet Spenser, whom I dare be known to think a better teacher than Scotus or Aquinas, describing true temperance under the form of Guion, brings him in with his palmer through the cave of Mammon and the tower of earthly bliss, that he might see and know and yet abstain.' 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.