David Parnas's Principles focus on software design and modularity, emphasizing the importance of separating a system into distinct components. This separation allows for easier maintenance, understanding, and testing of each module. Parnas advocates for clear interfaces between modules, which helps reduce dependencies and enhances the overall reliability of the software. Another key aspect of Parnas's work is the concept of information hiding. This principle suggests that each module should only expose what is necessary for other modules to interact with it, keeping internal workings private. By doing so, developers can change the implementation of a module without affecting other parts of the system, leading to more robust software development practices.