Here's why:

* It's an original document: The Declaration of Independence is the actual document created by the Continental Congress in 1776, expressing the reasons for the colonies' separation from Great Britain.

* First-hand account: It provides a direct, first-hand account of the events and ideas leading up to the American Revolution.

* No interpretation: It's not an analysis or interpretation of the events; it's the original statement itself.

While historians may study and analyze the Declaration, the document itself is the primary source, the foundation upon which further understanding is built.