We find this understanding very nicely stated by Thomas Aquinas in the thirteenth century:
Even in speculative matters there is something by way of work: e.g. the making of a syllogism or of a fitting speech, or the work of counting or measuring. Hence whatever habits are ordained to such like works of the speculative reason, are, by a kind of comparison, called arts indeed, but "liberal" arts, in order to distinguish them from those arts that are ordained to works done by the body, which arts are, in a fashion, servile, inasmuch as the body is in servile subjection to the soul, and man, as regards his soul, is free [liber]. On the other hand, those sciences which are not ordained to any such like work, are called sciences simply, and not arts.
I've actually come to think of this as rather important. Consider reading, which is as basic a liberal art as any. We tend to think of reading as in a sense transparent: you read what's on the page and that's that. But, really, anyone who does a lot of work with texts knows that it's not quite so simple, and you can have someone perfectly literate, in the sense of being able to read, who can look at a text and not make heads or tails of it, or, alternatively, who makes a complete hash of interpretation. This is because reading a text is not like just looking out a window; it involves work. Reading is doing things with texts, even if it's as simple as comparing this passage with that passage, or vividly imagining a scene, or making speculations as to what a character is thinking in light of what the author tells us. Good readers are people who have picked this up and practiced it enough that many of these things are second nature to them. This has made me realize that I need to change certain aspects of the way I deal with reading in my courses. Instead of just assigning reading, I should ask myself, "What can I have my students do with the text, given that I can take for granted that some students will have not picked up some of these habits? How can I modify my reading assignments so that instead of justing saying, 'Read,' I've made reasonably sure that they've had a chance to do something with the text -- and that will be a way to read it?" This is a very difficult set of questions, but I think we need to recover enough of the older notion of liberal arts as specific skills ancillary to intellectual life in order to ask them.
Similar things may be said of writing. I've long been dissatisfied with the role of writing at the college level, and in particular the pervasive reliance on the essay. There's nothing wrong with essays as such, but I think professors have difficulty remembering that essays are not natural means of expression. They are very difficult to do well; it's enough not enough to be able to write, you have to have at hand a rather sophisticated panoply of liberal arts, because essays don't just flow out of the pen, they are built, constructed using a wide variety of artistic tricks. In old-style rhetoric you would often practice something like essay-writing or speech-making by the method of emulation: you would build essays or speeches or whatever by copying great essays or speeches or whatever, extensively, so that you would pick up at least some of the art of it. And the essay is such an amorphous genre -- it was literally invented as a means of rambling, a form of 'miscellaneous writing', as an alternative to an actual treatise -- that if you just say 'Write an essay on such-and-such', there's no telling what you'll get. What we need to do is give assignments that clearly tell the students to do things with what they are writing about. Some teachers already do that; but I don't think most of us do, because we are usually the sort of people who easily took to writing essays, and so we forget that an essay is something that takes very specific skills of wordcraft.
The list could go on and on. The older notion of a liberal art as a skill of craft and work pinned down something very important about intellectual life, one that I think we have lost, although I think much of what we think of as 'good teaching' is based on at least a rough sense of this missing feature. It is also the only way, I think, to save the category of 'liberal arts' from being the miscellaneous flotsam and jetsam of the College of Arts and Sciences.