John Ousterhout's A Philosophy of Software Design is a masterpiece of software engineering wisdom. Here are some of the most impactful quotes from the book, organized by theme.

Complexity

Complexity is the heart of most software engineering challenges.

• Understanding Complexity: "If a software system is hard to understand and modify, then it is complicated; if it is easy to understand and modify, then it is simple."
• Change Amplification: "The first symptom of complexity is that a seemingly simple change requires code modifications in many different places."
• Cognitive Load: "The second symptom of complexity is cognitive load, which refers to how much a developer needs to know in order to complete a task."
• Incremental Complexity: "Complexity is incremental: you have to sweat the small stuff."
• Accumulating Complexity: "Over time, complexity accumulates, and it becomes harder and harder for programmers to keep all of the relevant factors in their minds as they modify the system."

Code Design

Good design is all about making the right tradeoffs and choosing the appropriate abstractions.

• Simplicity: "It’s more important for a module to have a simple interface than a simple implementation."
• Write Twice; Commit Once: "Designing software is hard, so it’s unlikely that your first thoughts about how to structure a module or system will produce the best design."
• Abstractions: "When designing methods, the most important goal is to provide clean and simple abstractions."
• Deep Modules: "The best modules are deep: they have a lot of functionality hidden behind a simple interface."
• Information Hiding: "Information hiding makes it easier to evolve the system."
• Avoiding Config Parameters: "You should avoid configuration parameters as much as possible."

DX (Developer Experience)

The humans in the software development loop are just as important as the "loops" themselves.

• Optimize for Legibility: "Software should be designed for ease of reading, not ease of writing."
• Reduce Suffering: "Most modules have more users than developers, so it is better for the developers to suffer than the users."
• Comments and Clarity: "The guiding principle for comments is that comments should describe things that aren’t obvious from the code."
• Obviousness in Code: "If someone reading your code says it’s not obvious, then it’s not obvious, no matter how clear it may seem to you."
• Choosing Good Names: "Choosing a mediocre name for a particular variable... will have a significant impact on complexity and manageability."

Taken together all of these quotes stress that software is as much about managing complexity as it is about writing code.