#SWIProlog comes with a very nifty IDE. start swipl myfile.pl
?- edit. You're in pceEmacs, an emacs clone running in your vm.
C-c C-n to get a project navigator, menus to get thread monitors, debug message window.
#Prolog is a 'closed world' system. If Prolog can't prove it, it's false.
OWL, reasoning for the semantic web, is open world, and assumes there might be more triples it doesn't know.
Closed reasoning is more powerful, but is, well, "closed minded".