There is no "The" ...

... before the words "libertarian philosophy."

While the words "libertarian" and "libertarianism" have a few centuries of history and meaning shifts, apparently beginning as a term to distinguish Christians who believed in free will rather than in predestination, for the last half century or so -- at least in the United States -- one particular meaning has gained overwhelming mindshare among those who use the word to describe themselves:

"Libertarianism" is the non-aggression principle, and "libertarians" are those who hold to that principle.

Of course, people get to the non-aggression principle via different (and possibly contradictory) routes, and proceed from the non-aggression principle to different (and possibly incompatible) elaborated sets of ideas. And of course any or all of those people may be in error at some point in that journey. "Libertarian" is not a synonym for "correct."

So when Sheldon Richman writes ...

I continue to have trouble believing that the libertarian philosophy is concerned only with the proper and improper uses of force. According to this view, the philosophy sets out a prohibition on the initiation of force and otherwise has nothing to say about anything else.

... my response is to notice the word "the" and say "well, there's your problem."

Every philosophy encompasses everything -- metaphysics, logic, epistemology, ethics and esthetics -- either explicitly or implicitly. The most fully elaborated, explored, formalized, systematic philosophies try to do so explicitly, but your personal philosophy addresses each of those elements, too, even if you've never consciously thought that personal philosophy through to figure out precisely how and why it does so.

Thing is, libertarianism is not a philosophy. It's a single constraint. "A" libertarian philosophy is a philosophy which incorporates that constraint. There is no "the" libertarian philosophy ... there are many libertarian philosophies, all of which presumably deal with matters other than proper and improper uses of force, using tools other than the libertarian constraint itself.