“There is only one good. And that is to act according to the dictates of one's conscience.”

Pennsylvania Constitution (1776), Declaration of Rights http://www.pahouse.com/pa_const.htm; this may be based upon principles stated by Penn, but earlier renditions in this form have not been located.
Disputed
Context: All men have a natural and indefeasible right to worship Almighty God according to the dictates of their own consciences; no man can of right be compelled to attend, erect, or support any place of worship, or to maintain any ministry against his consent; no human authority can, in any case whatever, control or interfere with the rights of conscience, and no preference shall ever be given by law to any religious establishment or modes of worship.

Source: Commonplace Book (1985), p. 76

The Influence of the Reformation on the Scottish Character (1865)
Context: It remains a lesson to all time, that goodness, though the indispensable adjunct to knowledge, is no substitute for it; that when conscience undertakes to dictate beyond its province, the result is only the more monstrous.
It is well that we should look this matter in the face; and as particular stories leave more impression than general statements, I will mention one, perfectly well authenticated, which I take from the official report of the proceedings:—Towards the end of 1593 there was trouble in the family of the Earl of Orkney. His brother laid a plot to murder him, and was said to have sought the help of a 'notorious witch' called Alison Balfour http://www.orkneyjar.com/folklore/witchcraft/balfour.htm. When Alison Balfour's life was looked into, no evidence could be found connecting her either with the particular offence or with witchcraft in general; but it was enough in these matters to be accused. She swore she was innocent; but her guilt was only held to be aggravated by perjury. She was tortured again and again. Her legs were put in the caschilaws — an iron frame which was gradually heated till it burned into the flesh — but no confession could be wrung from her. The caschilaws failed utterly, and something else had to be tried. She had a husband, a son, and a daughter, a child seven years old. As her own sufferings did not work upon her, she might be touched, perhaps, by the sufferings of those who were dear to her. They were brought into court, and placed at her side; and the husband first was placed in the 'lang irons' — some accursed instrument; I know not what. Still the devil did not yield. She bore this; and her son was next operated on. The boy's legs were set in 'the boot,' — the iron boot you may have heard of. The wedges were driven in, which, when forced home, crushed the very bone and marrow. Fifty-seven mallet strokes were delivered upon the wedges. Yet this, too, failed. There was no confession yet. So, last of all, the little daughter was taken. There was a machine called the piniwinkies — a kind of thumbscrew, which brought blood from under the finger nails, with a pain successfully terrible. These things were applied to the poor child's hands, and the mother's constancy broke down, and she said she would admit anything they wished. She confessed her witchcraft — so tried, she would have confessed to the seven deadly sins — and then she was burned, recalling her confession, and with her last breath protesting her innocence.
It is due to the intelligence of the time to admit that after this her guilt was doubted, and such vicarious means of extorting confession do not seem to have been tried again. Yet the men who inflicted these tortures would have borne them all themselves sooner than have done any act which they consciously knew to be wrong. They did not know that the instincts of humanity were more sacred than the logic of theology, and in fighting against the devil they were themselves doing the devil's work. We should not attempt to apologise for these things, still less to forget them. No martyrs ever suffered to instil into mankind a more wholesome lesson — more wholesome, or one more hard to learn. The more conscientious men are, the more difficult it is for them to understand that in their most cherished convictions, when they pass beyond the limits where the wise and good of all sorts agree, they may be the victims of mere delusion. Yet, after all, and happily, such cases were but few, and affected but lightly the general condition of the people.

Epilogue, p. 410
The Theory of Gambling and Statistical Logic (Revised Edition) 1977

Reported in Walter J. Miller, "Calhoun as a Lawyer and Statesman"' part 2, The Green Bag (June 1899), p. 271. Miller states "I will cite his own words", but this quotation is reported as not verified in Calhoun's writings in Respectfully Quoted: A Dictionary of Quotations (1989).

17 U.S. (4 Wheaton) 316, 409-411
McCulloch v. Maryland (1819)
Context: [T]he power of creating a corporation is one appertaining to sovereignty, and is not expressly conferred on Congress. This is true. But all legislative powers appertain to sovereignty. The original power of giving the law on any subject whatever is a sovereign power, and if the Government of the Union is restrained from creating a corporation as a means for performing its functions, on the single reason that the creation of a corporation is an act of sovereignty, if the sufficiency of this reason be acknowledged, there would be some difficulty in sustaining the authority of Congress to pass other laws for the accomplishment of the same objects. The Government which has a right to do an act and has imposed on it the duty of performing that act must, according to the dictates of reason, be allowed to select the means, and those who contend that it may not select any appropriate means that one particular mode of effecting the object is excepted take upon themselves the burden of establishing that exception. [... ] The power of creating a corporation, though appertaining to sovereignty, is not, like the power of making war or levying taxes or of regulating commerce, a great substantive and independent power which cannot be implied as incidental to other powers or used as a means of executing them. It is never the end for which other powers are exercised, but a means by which other objects are accomplished. No contributions are made to charity for the sake of an incorporation, but a corporation is created to administer the charity; no seminary of learning is instituted in order to be incorporated, but the corporate character is conferred to subserve the purposes of education. No city was ever built with the sole object of being incorporated, but is incorporated as affording the best means of being well governed. The power of creating a corporation is never used for its own sake, but for the purpose of effecting something else. No sufficient reason is therefore perceived why it may not pass as incidental to those powers which are expressly given if it be a direct mode of executing them.

The Rights of the Colonists (1772)

Quoted in The Independent, Sunday 17 October 2010 http://www.independent.co.uk/news/people/profiles/sousa-mendes-saved-more-lives-than-schindler-so-why-isnt-he-a-household-name-too-2105882.html

1770s, African Slavery in America (March 1775)