In the example pointed out before, the Shootings of May Third, is the depiction of a particular historic event. What raises this painting however to the level of art, is that also represents a universal truth about humanity. It is not simply that the painting depicts the scene of the firing squad, that makes this art, but that it depicts the real emotion, the defiant courage of the man in the face of death, that all men feel kinship to. This scene, which to a historian would be a mere fact, becomes through the focus on the executed man's courage, a vessel for communicating a universal truth to anyone who views it, regardless of having knowledge of the particular circumstances of this historic event.
The historians might tell us that such a painting is not entirely accurate depiction of the events of May Third, or even that this happened some other time, but to an artist this is unimportant. Rather than being interested in the accuracy of an event depicted, an artist is more interested in the truth, the universal truth of this man's courage. Indeed this is why fiction is so lauded, because it tells us more truth in the telling of a story than science might ever tell us. The novelist Patrick O'Brian points this out through the imminently philosophical scientist, Dr. Stephen Maturin.
But I imagine, sir,' - to Stephen - ' that you read books on medicine, natural philosophy, perhaps history - that you do not read novels or plays.' 'Sir,' said Stephen, 'I read novels with the utmost pertinacity. I look upon them - I look upon good novels - as a very valuable part of literature, conveying more exact and finely-distinguished knowledge of the human heart and mind than almost any other, with greater breadth and depth and fewer constraints." - Patrick O'Brian, The Nutmeg of Consolation.The art of literature then tells us more about the universal nature of man than any discourse on psychology or anatomy ever could, because it focuses on what is universally true about man, not just what is correct. Simply painting a man on his horse would tell us no more than a photograph might, but should that man be pointing and sternly riding a rearing horse, what is communicated is clearly the virtues of his great leadership and stern courage, which makes message of the painting not the man, but the virtues which can be universally known to all men. This is what makes art philosophical, that art speaks about parts of the soul of human beings which are common to all of us.
What makes something art is that it depicts not just the events and situation of a particular event or person, but that depicts the parts of about human nature which are universal to all mankind, and through that points to something other than the mere facts of our existence.