In earlier posts, Jason nicely laid out a variety of forms of philosophical liberalism and Jacob nicely indicated a difference between the philosophical and political.  Here, I'd like to briefly lay out my own view of what liberalism is, as a political view, and, even more briefly, the relationship between that and philosophical liberalism (in any version).  It may be that this should have been my first post.

It was the first classical liberals—perhaps most notably John Locke—who argued for significant limits to state power to interfere with citizens of the state.  This was part of a shift in thinking about people living under a regime as citizens rather than as subjects.  As subjects to a power, individuals could be interfered with at the will of the ruler.  As citizens, they could not.  As citizens, much of their lives were up to them and their choices—within some limits—had to be tolerated.  Liberals all accept this.  The debates amongst the different sorts of liberals (those committed to a political doctrine that is liberal) concern the proper limits of toleration.

Joel Feinberg–one of my intellectual heroes–says that liberalism is a commitment to (his version of) the harm principle and (his version of) the offense principle.  The harm principle indicates that harm to others is always prima facie justification for interference—in particular, state interference.  (As I read him, Mill takes the harm principle to apply far more broadly and I agree with him, but for our purposes, we concentrate on state interference only.)  In Mill’s original formulation, only harm serves that role (and I tend to agree).  As Feinberg deftly argues, the sense of harm that is in use in the harm principle is not merely a hurt, or a setback of one’s interests, but a wrongful set back of one’s interests.  The offense principle indicates that offense to others is always prima facie justification for interference with others.  I will say nothing more about the offense principle.

I think this is a useful way to begin to discuss liberalism as a political view, but I think we should recognize that there are different sorts of  liberalisms in the political arena.  Liberalism is actually a family of views.  On my own view, the harm principle does the work of indicating the normative limits of toleration and so also the limits of justified state action (with a possible exception that I may post about in the future).  On Feinberg's view, its the harm principle and the offense principle.  Other liberals include what we may call a benefit-to-other principle: benefit to some is always prima facie justification for interference with others.  Again, I will say nothing more about this principle.  (Though I should note that Rawls's difference principle is not a version of it.) 

Historically, I should note, there are two other principles that get much attention: the principle of legal moralism (harmless immoralities are prima facie reason for state interference) and the principle of legal paternalism (prima facie. state interference is justified to prevent an individual from hurting herself).  Some who claim to be liberals actually endorse the latter even today.  I tend to think both of these principles have to be rejected.  I think its reasonable, though, to call "liberal" anyone who endorses the harm principle or the harm principle and the offence principle or the benefit-to-other principle or both.  Libertarians likely accept only the harm principle.  Libertarianism, in my view (some notable figures disagree) is a form of liberalism.

Liberalism–and libertarianism–as discussed above is a political view.  Philosophical liberalism has much to say about liberalism, the political view.  Philosophical work is needed when discussing the political for at least 2 things: (1) defending the political view being offered; this includes showing what ultimately grounds the political view; (2) making the political view itself plausible; this includes working to be sure it is internally consistent and also to be sure that it does not require the rejection of some known scientific (esp. psychological and economic) fact.  Both of these are important tasks.