I haven’t read much Karl Marx since the early 1980s, when I taught political philosophy at the University of Pennsylvania. Still, it didn’t take me long this weekend to find my copy of “The Marx-Engels Reader,” edited by Robert C. Tucker — a book that was assigned in thousands of college courses in the 1970s and 80s, and that now must lie, unopened and un-remarked upon, on an awful lot of rec-room bookshelves. My occasion for spending a little time once again with the old Communist was Barack Obama’s now-famous comment at an April 6 San Francisco fund-raiser. Obama was...