Article 1, Section 6, Clause 1
James Wilson, Legislative Department, Lectures on Law1791Works 1:421
The members of the national legislature, and those also of the legislature of Pennsylvania, shall not, for any speech or debate in either house, be questioned in any other place. In England, the freedom of speech is, at the opening of every new parliament, particularly demanded of the king in person, by the speaker of the house of commons. The liberal provision, which is made, by our constitutions, upon this subject, may be justly viewed as a very considerable improvement in the science and the practice of government. In order to enable and encourage a representative of the publick to discharge his publick trust with firmness and success, it is indispensably necessary, that he should enjoy the fullest liberty of speech, and that he should be protected from the resentment of every one, however powerful, to whom the exercise of that liberty may occasion offence.
When it is mentioned, that the members shall not be questioned in any other place; the implication is strong, that, for their speeches in either house, they may be questioned and censured by that house, in which they are spoken. Besides; each house, both in the United States and in Pennsylvania, has an express power given it to "punish its members for disorderly behaviour." Under the protection of privilege, to use indecency or licentiousness of language, in the course of debate, is disorderly behaviour, of a kind peculiarly base and ungentlemanly.
The Founders' Constitution
Volume 2, Article 1, Section 6, Clause 1, Document 16
The University of Chicago Press
The Works of James Wilson. Edited by Robert Green McCloskey. 2 vols. Cambridge: Belknap Press of Harvard University Press, 1967.