"LOOK TO NATURE TO LEARN HOW REALITY WORKS."
While I believe the entirety of the natural world has a lot to teach us, this Principle focuses mostly on human nature. Specifically, it says that understanding how the human brain has evolved allows us to better appreciate how we view and react to the world.
Dalio writes, "Most people struggle to reconcile their emotions and their instincts with their reasoning." Of course we do! Different parts of the brain developed under different circumstances. The instinctual part of the human brain evolved to survive by avoiding threats and seeking caloric efficiency. This is why we dislike criticism and tend toward procrastination. The intellectual part of the brain evolved to cleverly hunt prey and protect the tribe.
By first understanding our instinctual programming we can give ourselves a better chance of using the more recently developed parts of our brain to overcome these tendencies. We handle tough feedback and show vulnerability because we know it allows us to grow.
On a side note, nature is a great place to look for sanity and inspiration. And as much as most people like to define nature as trees, and lakes, and mountains, really there is no natural/unnatural duality. Even the man-made is natural. If you break anything down to its smallest parts you're left with a whole lot of nothing - just atoms, filled with empty space, bound together in different combinations to form DNA, or an Octopus, or a building.
So sometimes I think about that, and it makes me feel immensely small and big, meaningless and powerful all at the same time, which is a nice way to feel.