[I add a few more comments and reference a new book on Justice Scalia here.]
Recently, there was some talk on the Slate fray about Jefferson, and the value of referencing him in respect to original understanding of the Constitution. It was noted -- somewhat exaggeratedly -- that he really did not like the document very much, and especially was not a fan of a strong judiciary. I would note on that front that he was the one who counseled Madison that a Bill of Rights was necessary, one which the courts would have a major role in upholding. And, especially with a strong Tenth Amendment, the Constitution was not too bad in his eyes. But, yes, he had some problems with the whole thing, and was no fan of a strong judiciary. Anyway, the general sentiment of some was that he was not a good person to use, partly because Jefferson was not a Framer.
The best way to answer critics on this front is to rely on original understanding. Original understanding is a special aspect of originalism, actually the most workable form -- see, e.g., Randy Barnett's writings (libertarian view). In this form, we look at the general understanding of constitutional terms and principles in the Founding Era. Thus, when trying to determine the meaning of "freedom of speech," we do not only look at the original intentions of the Framers (trying to be mind readers) or original meanings per se (perhaps channeling Noah Webster), but taking a more holistic approach.
After all, "the people," ultimately ratified the Constitution ... and they were its target audience. And, if you look at originalist court rulings, original understanding overall is often the concern, though meanings also are used with relevant citations of dictionaries and such. Looked at in this fashion, Jefferson is quite relevant. He was the face of the Democratic-Republican Party, which promoted a certain constitutional vision. It is quite logical, therefore, to consider such an influential voice -- the primary author of the Declaration of Independence to boot* -- an important reflection of original understanding.
Furthermore, a bit of looking will show that the themes he promoted -- including that famous letter to the Danbury Baptists respecting the meaning of separation of church and state (see, e.g., The Godless Constitution) -- was not unique to him alone by any means. Again, a reflection of original understanding. [Or, to be even more specific, many of Jefferson's opinions can be shown to be reflective of actual original intent/meaning.] Surely, it is foolhardy to look to any one person, or even one point of view, to determine original understanding. For instance, in John Witte Jr. in Religion And The American Constitutional Experiment notes that four strands of thought was in place respecting the proper division of church and state in the Founding Era.
Thus, though certain basic principles were clearly agreed upon, tricky situations arise in which no one "original" understanding can be found -- it ultimately would be a judgment call. Still, certain lights are logically looked upon with special favor, and Jefferson is correctly one of them. Finally, hard as it might be for some to imagine, I actually respect original understanding; I just think it is more flexible and open-ended than some think. John Marshall, a ratifier of the Constitution, reminded us that this is a Constitution, not a prolix legal code. It was intended to be a flexible and developing document (up to a point).
We should take a page not only from TJ's book, but that of his nemesis. Both have something to teach us.
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* And not only that. Jefferson worked with Madison, the latter actually a major framer of the Constitution / Bill of Rights, for a religious freedom law in Virginia that clearly was an inspiration of the First Amendment. He also worked on the Northwest Ordinance, a major piece of legislation in the 1780s setting forth a means of self-rule and future statehood for Old Northwest, the current Great Lakes Area.
This influenced constitutional rules respecting territories and future statehood. Furthermore, TJ was the nation's first Secretary of State, providing a major platform to establish precedents concerning what the Constitution truly means in practice. He might have been France while the Constitution was being written, but TJ was a major player and a good place to start in respect to original understanding.