I've been reading other things, things that I simply haven't had the inclination to blog, but whilst reading about some controversies in social science I realized that one of the things that Snow used to demarcate the line between science and humanities can also demarcate a line between social and natural science: The cumulative nature of knowledge.
I should state at the outset that I do, in fact, acknowledge that social scientists have cumulative knowledge, that they do, over time, develop and refine a body of generalizable knowledge about human beings. That's not to say that every theory or every finding turns out to have wide applicability (or even replicability), but the process of social science can indeed select, filter, and refine the body of knowledge, just as natural scientists do.
In the sense that social science can produce a refined, tested body of cumulative knowledge, social science is akin to natural science, and distinct from arts and humanities in the sense that Snow articulated.
On the other hand, because social scientists study humans they have to confront human prejudices and cultural inertia, and thus they have to re-fight certain battles in every generation. Topics like, say, gender differences, get re-fought in each generation, and the cumulative weight of data will not dampen the appeal of gender essentialism as a way of legitimizing inequality. (One could easily come up with other perennial battles; I simply picked that one for the sake of easy illustration.) In that sense, there is a human limit on the extent to which social sciences can, in practice, be as cumulative as natural science, or at least a limit to the rate of accumulation, because of the need to re-fight battles in each generation. There is much less of that in natural science. OK, evolutionary biologists have to re-fight battles in the public sphere, but not within academia. And biologists studying development, cognition, and gender will have to join social scientists in re-fighting gender battles in each generation, but that reflects their position at the intersection of natural and social science. Closer to (my) home, we physicists have to help each cohort of freshmen overcome Aristotelian intuitions about motion, but that battle really only takes a semester. By the end of the semester they know that they ought to be Newtonians; they might still have Aristotelian impulses, but they know that they're supposed to check those impulses.
So, if we take the cumulative nature of natural science as a line of demarcation between Snow's "Two Cultures" then I think a tri-partite division of liberal arts is appropriate: The humanists can generate new works of art and inspiration and analysis in each generation of a changing world, without the constraints of the past, the natural scientists develop a cumulative understanding of nature, and the social scientists seek to build a cumulative understanding of people but do so while re-fighting perennial battles of human culture and prejudice.