I. Our Constitution Is a Concept that Includes Inalienable Rights
A nation is constituted by a set of ideas that define it politically and legally. That constitution is a concept, typically gray in details, not just a document that may presently express all or part of the concept. Most nations' main constitutional features do appear in one document. But the constitutional status and exclusivity of such texts remain ever subject to their current political and legal legitimacy. In 1788 we simply stopped treating our "Articles of Confederation and Perpetual Union" as a continuing authority, though that text's own terms precluded its being replaced through the 1788 Constitution's adoption process. And document titles alone do not decide. The United Kingdom is constituted without any document called "Constitution." Conversely, a nation is not simply assumed--without more evidence--to be exclusively constituted by terms of any document that bears the name "Constitution."
Habits of thought and assumptions born of media rhetoric blind many Americans to our full constitution. Most habitually just assume a certain role for the 1788 text, without further reflection. More clear-eyed scrutiny of that text's role begins with contemplating the reasons for following texts in general.
The Authority of Constitutional Text Comes from the People, Not the Text
Justification for assigning authority to a text cannot be merely a command to follow from the text itself--not even a command from a text claiming to be religious revelation or a country's constitution. Otherwise, Christians would also follow the Koran; every religious person would follow it, the Torah, New Testament, and other religious tracts claiming self-authentication. And for a constitution, Americans might still follow our Articles of Confederation.
Hence, reasons for following a text typically lie outside the text itself. And such external reasons influence not only why we read texts but also how to read them and whether also to look to other texts or sources.
We understand how a personal faith, outside text, leads believers to find religious significance in particular texts--a faith perhaps blending internally-felt divine inspiration with trust in one's family or social group's embrace of the text, independent learning, or desire for mental peace. That faith not only determines which texts are sacred but affects the strictness with which a text is understood.
In each nation, something like a faith--a politically-conjoining, collective assent by the living citizenry--is what justifies which texts or other sources comprise the foundation of law and government, what reveals the nation's present constitution. Though vague, that collective faith provides the "standard" (rule, principle, process) by which the people recognize what constitutes their nation in a political and legal sense. The American "constitutional standard" convinces us that our constitution still includes the 1788 document. True, many Americans do not consider the reason but simply follow a parent, teacher, or pundit who calls that text "The Constitution." But even if we seldom ponder our constitutional standard, on reflection, Americans appreciate that more than an authority figure's bare assertion must justify turning to a particular document.
Though additional details of America's constitutional standard are debatable, we do agree on the principle at its core: our varying ideas of what makes law and government legitimate all concur that (i) the peoples' democratic will and (ii) the individual's inalienable rights are co-requirements for our constitution. Those two requirements make the 1788 Constitution the central component of--but not the entirety of--the American constitution today.
Parts II and III, below, elaborate historical reasons why popular sovereignty has made some precedent also part of our constitution; but the inalienability of certain rights is another, simpler reason the 1788 Constitution cannot have been exclusive, from its very inception:
Our Inalienable Rights Are Not Limited by Text
inalienable Not transferable to another or capable of being repudiated.
Even "constitutional conservatives" want rights beyond text. In addition to the 1788 document, they embrace our 1776 Declaration of Independence and its principle of "[i]nalienable rights." ("Unalienable," in official Declaration copies, meant what Jefferson's draft called and we now call "inalienable.") But in alienable rights cannot have been alienated (given up, taken away) by political majorities' failure to vote them into a later document like the 1788 text or Bill of Rights. And majorities cannot be relied upon to adopt or amend constitutions to protect the very minority rights they infringe. Thus, some rights against government--constitutional rights--reside outside the 1788 Constitution's text, as the Ninth Amendment emphasizes: "The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people."