So, I've come across this concept which, might I say, may be the most important thing I've learned from my professor.

I doesn't matter if you are a realist, impressionist, abstract, or surrealist, everything is abstract. Not even what medium you work in, be it photography, graphite, or oil color. A realistic painting or drawing is nothing more than a collection of abstract marks and shapes come together to create an image. So don't treat it anything like wood, or like skin, or metal. Treat it like an abstract work, that when shown in conjunction with other abstract shapes, conveys the idea of the object you are showing the viewer.