First Principles as Constraints
We need Constraints. It helps us develop faster, by narrowing our focus and allowing us to have a box to stand on, from which we can start thinking about what lies out of the box.
Constraints can be found through First Principles Thinking. When you break down something to its core, you take the effort to understand what you can do with it, and what you cannot. The also allows you to ask questions regarding why something has worked, or what has worked and further expand into the question of will it work for the conditions that are yet to come and how should I transform it to better suite the needs of today. These are important questions to ponder. The answers to these questions will form the basis for your constraints when developing a new idea or a product. Using the constraints you can iterate and innovate to build something completely new from the group.
I think it is okay to consider Constraints as a subset of First Principles Thinking.