Now in all the above the other parts - such as mere breath, or that material which is insensate - have no direct affinity to each other: yet even here a link is formed by a sort of unity and the gravitation of like to like. But the mind has this unique property: it reaches out to others of its own kind and joins with them, so the feeling of fellowship is not broken.
In this beautiful and eloquent passage, Marcus observes the light of the sun and how it is one light all over the world, despite being broken by objects. He applies this idea to one Directing Mind - one animate, intelligent soul, that even though it appears divided by the millions and billions of people, is still unified.
He then observes the same concept with the human body - how the breath, the arms, the legs, etc, all function as one. And the one thing in common is the mind. And of all the human parts, only the mind will reach out to other minds and it creates this link with other people. Such is our social nature and as such, we should live according to nature.
(see also Citadel p. 113, 260)