What is Bertrand Russell’s third passion?

Bertrand Russell wrote a short and sweet essay called "Three Passions I Have Lived For." Here are Russell’s three: The longing for love; the search for knowledge; and what he called "the unbearable pity for the suffering of mankind. "Of these three, the first two require no external reminders. One seeks love because love brings ecstasy and relieves loneliness; one seeks knowledge to understand the human heart. It is the third passion, the unbearable sympathy for the suffering of others that seems to call for frequent updating. So the world accommodates, with glimpses of that unbearable sympathy in Africa, where among the spasms of slaughter and disease, volunteers labour to keep that tortured continent alive; or in Iraq, where, between the pictures of invasions and explosions, a soldier occasionally is spotted lifting a child to safety. Or within our own borders, where those without homes or food are attended by those who refuse to let them go under. That constitutes the "unbearable" part, passion undercut by the powerlessness. Of course, reason and gentleness are politics by other means as well, but they require a different sort of passion. Russell’s ideas of passion were straightforward and in the right place, which made his life worth living.