It's not a concept that was somehow introduced with post-modernism. It has always been here since subjectivity is a fundamental characteristic of art. Even more, it's a fundamental condition : if your assessment of a work is not of a subjective nature, then that thing can't be art (or at least, not in your eyes).

Design a building. Design a house. Write an album. Sing a song. Draw a line. All of those are tasks/art wherein someone sets out to produce something. Some people produced Hit Me Baby One More Time and some people produced Sgt Pepper's Lonely Heart's Club Band. Van Gogh made Still Life with Cabbage and Clogs, you or I would make a crappy building with whacked out perspective. There is objective value to art.

I absolutely believe there is a skill involved that can be seen, observed and measured.

There are carpenters that can make a table at which you and I can sit to eat our lunch, and someone else will make a table that collapses when we put our plates down.

Take a painting like Malevich's red square. How are you going to argue to someone that it is great art ? You can't, since there isn't much to discuss, really.

Because what you or I or my neighbor would set out to make would not have that amazing, deep shade of red. It would look like a crayola marking.

With movies, since they are of a much higher complexity than that painting,

I disagree.

it is possible to build some argumentation which may help someone to find some appreciation for the movie that they didn't initially have.

Correct. And those are observable rules by which we measure art.

But it's still just a combination of things which, ultimately, have to go through the subjectivity of that person, who will be the sole arbitrer on the quality of the work. In the end, everyone has to think for themselves and find out what they like and what they don't, rather than blindly follow some established cannons and filmmaking rules.

I agree we all have to think for ourselves...however, please try to put forth the argument that Gigli is a better crafted film that Lawrence of Arabia. I dare you. You cannot! Lean's craftsmanship is arguably second to none. Lean may very well be one of the greatest artists of "the cut" I've ever seen.

I can say that with near absolute confidence because:

I don't believe in post-modernism

It's the truth, and you know it! :)