Despite being written more than two centuries ago, the United States Constitution continues to be one of the ultimate authorities on American law. The fact that it is subject to differing interpretations over time, and that the Constitution changes, renders it a "living document." This concept, known as the "living Constitution," is popular, but nonetheless presents some key disadvantages, both practical and philosophical.
Lack of Substantiation
The single biggest philosophical disadvantage of the Constitution as a living document is that there's no real substantiation for any given point of view. Suppose a law is proposed that would ban the use of cell phones by cloned human-robot hybrids. Because the original writers of the Constitution, known as "framers," had no experience with cell phones, cloning or robots there's no way to know what they would actually have thought; any attempt to say what they would or would not have intended in the Constitution is pure speculation.
Practically, the Constitution as a living document also introduces inconsistencies. If the Constitution is seen to change with the times, then what was constitutional 10 years ago might very well not be constitutional now. By that same token, it might be constitutional again 10 years from now. Allowing the Constitution's meaning to change has its upsides, but it also means that the Constitution becomes unreliable as a way of determining what's legally appropriate.
Undermining the Authority of the Constitution
A "living Constitution" also continuously undermines its own authority. If the real intent of the Constitution is not to be found in the words themselves, but rather in how those words are interpreted at a given place and time, then the written Constitution has no meaning. For people opposed to the Constitution as a living document, a position known as "originalism," this seems to say that the Constitution might as well be a blank piece of paper on which each successive court can write their own opinions.
By allowing the courts to create their own interpretations of what was intended by the Constitution, viewing the Constitution as a living document also fosters "judicial activism," the phenomenon in which judges appear to rule outside the law. If they can interpret the Constitution how they like, originalists argue, judges are given far too much power. In effect, a living Constitution makes the Supreme Court the ultimate authority in the United States because that court is in the position of deciding what the Constitution is supposed to mean; this removes the power of the legislature and the American people.