Just as the nature of the Whole is the source of all other faculties in every rational creature, so it has given us this power too. In the same way that nature turns to its own purpose anything obstructive or contrary, placing it in the fated scheme of things and making it part of itself, so the rational being can also convert every obstacle into material for his own use, and use it to further whatever his original purpose was.
Marcus takes the view that humans are to be as a whole, just a body is not whole unless it has a head, two arms, two hands, two legs, two feet, and a torso. He witnessed savage war and saw heads and body parts severed and laying a part from the body. And he draws the comparison of a body part severed from the body to a human being separating himself or herself from society. The wonderful thing about being human is the ability to separate oneself from society and still have the ability to rejoin it. You, as a human, were meant to be a part of society. If you are apart from it, you can still rejoin it - count this as a blessing!
Humans - rational beings - have the ability to make events work to their advantage. Events happen, other human beings act contrary to what we expect. We do not have to be frustrated by these "externals" (things entirely out of our control). Rather, we can accept them for what they are, and pivot. We can pivot in action or we can change (also a pivot) our attitude about those events. This is how "the rational being ... convert[s] every obstacle into material for his own use, and use it to further whatever his original purpose was." Let me give one example, from my life, of this mindset.
I used to really, really love playing basketball. I knew I was not the best, but it didn't matter. As long as I got to play, I was content. I played 3 to 4 times a week with my dad and friends. I made the freshman, sophomore and junior varsity teams in 9th, 10th and 11th grades. Between my junior and senior year, I practiced a lot and went to a basketball camp. It was my aim, that year, to make the varsity team and play. I tried out, practiced hard, hustled and made every effort possible. My coach recognized the effort, but in the end, decided I would be cut. Knowing what my real intentions and desires were (just wanting to play basketball), I asked the coach if I could still practice with the team. He allowed it. I later realized it was the perfect scenario for me! I got to practice, play basketball without any pressure, and then sit in the stands with my friends and cheer on the team. I also was able to play in another league, outside of school, and found it extremely rewarding. At the moment I was cut from the basketball team, I could have been very sad and griped and complained about this obstacle. But knowing the real desires in my heart, I pivoted and looked for a way to turn this obstacle into my advantage.
I've heard a quote before, sometimes attributed to Michael Jordan, which goes something like this: "I've either won or learned." It's that mindset of "always winning and never losing" that looks at so-called obstacles and makes those obstacles work to our advantage.