What is the core concept of the spiritual self?

Men must strive to elevate themselves. They can only do so by working out good effects. If their purpose be to benefit each other, they will develop the germ of charity. They must strive to throw off the material shackles of selfishness, which they have nursed so long with disappointed hopes. They must unite in sympathy with each other. They must extend the hand of fellowship, and especially he who has attained a certain degree of individuality, it is his duty to strive to comprehend that the true way to progress is to assist his fellow being in the same path.

―B. H. In Spirit