What is the content of an impressionist rendering? Little more than value masses and their joints. It can't be put more simply than that. We see, we perceive the world, in value-produced effects. Rendering value-effects as they occur before our eyes produces visual likenesses of anything or any group of things we see. Value-effects exist in degrees of value, contrast, and abruptness of edge, and thereby produce varying effects. To create a likeness, all we have to do is collate the values and effects of any ensemble of such units accurately in the correct order of their strengths, sizes, and locations on the picture plane. Having done so, we will observe that we have not only created the appearance of objects, but the illusion of three dimensional space and the atmosphere of that space - every bit as critical to the complete and wholistic expression of the visual appearance.
To accomplish such, it is of foremost importance to jettison the outlining of objects – what Stevenson terms “primitive drawing” in his essay on Velasquez. As painters seeking mastery of the whole truth we must let go of that that seemingly "precious" tool. In clutching on to it, we block and mangle too many other visual realities. In our addiction to outline, we fail to discover the better way to bring the visual world into being. Instead we must endeavor to follow advice like Sargent's: to reproduce effects, spots, or points of contrast - and pursue their associated angles in relation to one another. Similarly when working in color-values, search out the color of the spots. Do this in willful obliviousness to any connection between them and any apparent, delimited "object." We also find ourselves perfecting the points of effect by the more or less simultaneous incorporation of color.
In this new and more complete paradigm, the painter must be expert in the observation of the relationships between these effects - effects considered not only in terms of their strength and intensity, but in their size and location as well. We must become adept in perceiving ways to render the “thing as a whole,” seeing and rendering systems or sets of smaller groupings by color, by value, or by effect - and the shapes created by their relative locations. For anyone dedicated to “realism” this will admittedly require a leap of faith. Yet as Gammell liked to point out, “you have nothing to lose but your chains.”
At no point is Velasquez in the included illustration painting a cat. He paints visual effects and visual relationships, visual impressions. The successful result reveals a cat - and more. The basic mastery for successful Boston School painting is over that which makes up the “look of nature,” its appearances, which may be effectively rendered as paint on canvas - nature and canvas having nothing in common except that which might emerge from a tube of color. As opposed to actual realities, we employ visual, and thereby paintable, phenomena - and more importantly, their relationships. The basis of the Boston School way is therefore applied mastery in the reproduction of these phenomena in paint. We must be good at rendering form by the gradation of values; of color notes with their value, hue and chroma; edges or what happens when values meet. We must acquire proficiency at producing true proportions and other relationships as they pertain to these things as they occupy the picture plane. We must acquire fluency in the relationships of colors to each other. Forms to each other. Angles to each other, light effects to each other, sizes, values and so on. And leaving off trying to make the shape of “things,” we must expand the shape concept to include not only the shapes of darks and lights - but that of the “figure created by the leading lines of the composition” and much much more indeed.
Though they exist wherever we look, we leave off trying to find out how to make things-as-objects, and don't even wonder how. We find that this "wondering how" drives us back into our heads in fruitless pursuit of a priori generalized information, rather than into our eyes in search of a world that is far too unpredictable to fit that box. "How to paint an apple" or "how to paint a portrait" would undermine our primary activity, the authoritative rendering of paintable visual phenomena and any internal or external relationships in terms of their size, location and effect as seen in a world reduced to a picture plane. Ultimately the person who can best paint from their head is the one with the greatest experience painting truthfully what their eyes see, comprised of the color notes first, and following that, their placement and other related visual concepts. We want to be able to paint what we see with our own eyes - not with our ears, what we may have learned in books or what people have told us - and to articulate this on its own terms and in ways most efficient at getting the job done. That is simply a matter of color-values and what happens when and where they meet.