While flattering, Rembrandt's portraits also exhibited a stark realism relatively new to the art world of grandiose, saintly images of idealized bodies in perfect poses. Rembrandt's self-portraits show him as an awkward-looking weasel of a man with wrinkles on his face and a wart on his nose. The honesty of such rendering speaks to the values of a man who wanted to paint people as they really are: flawed. Art will only begin to fully accept this vision of flawed humanity two centuries later with the advent of Modernism. But early spokesmen, like Rembrandt, Goya, and others who painted non-idealized images (some even downright grotesque) were the precursors to the ensuing trend of the second half of the nineteenth century into the twentieth century. In that sense these artists were very literally ahead of their time.
The thing I personally appreciate the most about Rembrandt's portraiture style is his ability to visibly capture the invisible. A portrait, if you think about it, is largely a secularized image of an individual. We are presented with the person's fleshly body, their outer image and appearance; but in such a two-dimensional rendering we certainly cannot tap into the person himself, his thoughts, beliefs, aspirations, and feelings, can we? Rembrandt is somehow able to. In his portraits we connect with the individual on a much deeper, emotional, and spiritual sense than other portraitists of previous artistic periods (perhaps with exceptions like da Vinci). Rembrandt is able to tell us something personal about the sitter in addition to presenting the viewer with that individual's physical qualities. Like Hals' Laughing Cavalier, emotion and expression of inner thoughts begins to come out more in portrait painting at this time—thanks to the Protestants.
This is the idea in art of the authoritative sincerity of the painter. I heard this concept lectured on in a certain museum about a year ago, and it may be hard to grasp; however, we must tackle it now. When Rembrandt paints a self-portrait, we believe that what we see is the reality of Rembrandt's image, whether it be an accurate likeness or not in the terms we would qualify as accuracy—i.e., whether the clef in his chin was really so large, the dimple on his cheek so measured in length, and so forth. We can forsake this image of Rembrandt as he would appear in the flesh and substitute it with his canvas creation because, under the authoritative, autographed name of the artist himself, his self-portrait is published to be Rembrandt. It's a step further from mere suspension of disbelief because it becomes the reality itself. Where one might say that by painting Rembrandt, Rembrandt has "put a little of himself" into the painting, a true art critic might say, "No, no, the painting is Rembrandt." The abstract soul of the artist is totally infused into the work, having been painted on a literal canvas, making the imaginary real. Now, the ideas of Rembrandt's portrait, of his intangible character traits and spiritual personality, are transferred from immateriality to physicality in the form of oil paints applied to a tangible canvas. This painted rendering of Rembrandt has become Rembrandt, the true version of the man (art succeeding the artist). So, when we look at a Rembrandt self-portrait, we completely believe that we are looking at the true face of the artist as he actually existed in the deeper, hitherto-imperceptible, philosophical vision of Rembrandt as the true soul he was and is evermore. We look at his soul when we look into the framed painting hanging on the wall in the gallery: it's not just a painting. At any rate, that is the idea.