Similarly, Santayana’s aloofness from political and social duties, and in general his avoidance of communal activities, is explained, or at least symbolized, by his adherence to the kernel theory. It may also account for his tolerant attitude toward the lives that others chose, and his more or less egoistic concern about his own. A person who is secure within his or her kernel need not feel threatened if different people experience the world differently. That only means that their kernels are not the same as our kernel. And neither is ours necessarily in conflict with theirs, or inherently preferable to them. Likewise, one can help other people fulfill themselves not by directly contributing to their welfare, as the spiral theory would suggest, but merely by refusing to impose restrictive outer layers. Because no one can change the inner core, we do best to leave it alone. This, in turn, increases the likelihood of being let alone oneself.
In this context Santayana goes so far as to admit that he is both selfish and heartless in his attempt to “resist human contagion, except provisionally, on the surface, and in matters indifferent to me.” Still, as he also points out, his selfishness was never competitive or aggressive.
— Irving Singer, George Santayana, Literary Philosopher
The "kernel" theory here refers to a sense of self centered on some irreducible essence, whereas the "spiral" theory describes the view that the self is merely the sum total of experiences. As a fellow traveler of Spinozan pantheism, I'm inclined to see the spiral version as more accurate in an ontological sense, but in practice and temperament, I can certainly relate to this passage.