The author introduces a principle that emphasizes the importance of keeping an object's behavior unchanged after its construction. This approach leads to code that is easier to use and less prone to bugs. They provide examples of violations of this principle, where methods are called on an object to modify its state after it has been created. Such practices can introduce confusion, especially when dealing with options that affect how an object operates. The example of a `DirectoryWalker` illustrates the pitfalls of changing configurations after adding folders. The initial code allows for option flags to be set after the folders are added, leading to uncertainty about how these options apply. The author suggests a clearer approach where options are set at construction, ensuring that behavior is predictable. This structure makes it easier to refactor code without unintended side effects, as each object's state remains consistent throughout its lifecycle. Ignoring this principle can lead to messy code, where setup functions become scattered and intertwined with various parts of the program. This can create situations where changes in one area inadvertently affect others, making debugging a challenge. The author points out that many objects lack proper documentation, complicating the process of understanding how they should behave, especially when initialization is unclear. There is a noted exception for procedural objects, such as a stream parser, where changing settings mid-use may not make sense. In these cases, the author prefers to create separate objects for different settings, which can simplify complex code. This approach can enhance clarity by reducing the need for constant setting adjustments, ultimately making the code easier to manage.