I do have one caution about Jack's appeal to what he calls the "underlying principles" of the text. When the text is vague, appealing to the underlying principles to determine whether or not it covers a particular situation is appropriate and inevitable. But what is not kosher is to dive beneath the surface of the text to ferret out the "underlying principles" and then resurface somewhere else entirely. This is a standard technique by which the text itself can be replaced with the interpreter's version of the "underlying principles" that may even contradict the text itself.
What is appropriate is using the principles underlying the text to interpret its reach, when that reach is in doubt. In other words, when you dive beneath the surface to find the "underlying principles" you must reemerge in the text itself and apply the text (not the principle) to the facts of a particular case.
Here is another way to put the point: All wordings can be vague depending on the situation in which it is being applied. But where the text is not vague, an appeal to underlying principles should not be used to avoid the application of the text to the facts at hand.
For example, that an "underlying purpose" of the Second Amendment (as evidenced textually by the preface to the right to keep and bear arms) was to preserve the conditions that made possible a well-regulated militia MIGHT be used to influence the application of the right to some factual situations at its margins. But that underlying purpose cannot properly be used to undermine the existence of the right itself where the application of the right is clear. Or the fact that an underlying purpose of the First Amendment was to protect political speech should not be used to undermine the protection of political speech on the grounds that this purpose is best served by restrictions on the political speech of some in order that others better be heard.
Among other reasons for rejecting this move is (1) that the right enunciated in the text is likely to be under- or over-inclusive of its underlying principle, but this was the textual means chosen to accomplish this end. (2) There may be other purposes served by the textual provision. And (3) the exact scope of the underlying purpose is highly uncertain and subject to manipulation by interpreters.
I do not think that Akhil Amar's appeal to "first principles" is committing this move. While I am not sure whether Jack is actually doing this--in fact it looks like he is not--certainly others do. Larry Lessig's "translation theory," for example, can easily be used this way and has been.
My purpose is to identify a potential switch away from original meaning in the ostensible name of original meaning by appealing not to the text, but to the unwritten principles underlying the text, and thereby obviating the text itself. This is a subtle move that an originalist committed to a written constitution should be aware of and should resist.
Update: A few brief replies to comments before I must run off to class. The Court admits that it's so-called "Eleventh Amendment" jurisprudence is not based on the text of the actual Eleventh Amendment. If the precedent of Hans v. Louisiana (1890) (decided long before the Rehnquist Court) was wrongly decided, so be it. As Akhil stated in his Slate essay, it is entirely proper to criticize originalists for inconsistency with their method. It is an important virtue of originalism that it permits an appeal to evidence so as to detect inconsistency. In this regard, other methods of "interpretation" may well be nonfalsifiable.
As for the Second Amendment, I (not Eugene) said in my original post that underlying principles CAN be used to deal with problems of vagueness requiring constitutional construction. I contended only that they cannot be used to contradict the text. And yes, this is very similar, if not identical to, the parol evidence rule in contract law that permits extrinsic evidence to be used to interpret--but not to contradict--the meaning of a written contract. Ambiguities (more than one meaning) too can be addressed by historical evidence, but are usually easily reconciled. It is vagueness (whether a term encompasses a particular object) that is the more serious problem.
One final thought: Originalist meaning textualism is not without its difficulties because the attempt to bind governments by written constitutions has inherent difficulties. The important questions are (a) how these difficulties compare with the difficulties of rival approaches to interpretation, and (b) whether the benefits of a written constitution can be maintained when the very persons who are supposed to be bound by a writing can change its meaning to something they think is superior.