Art teachers often regard the techniques and discipline of observational drawing to be a fundamental skill for the preparation and production of artworks. To be skilled in observational drawing is to have acquired a range of sophisticated competences that only highly intelligent creatures are capable of learning — skills that have gradually emerged through cultural evolution and that in many cases have only been discovered in very recent history. Observational drawing is one of these remarkably late additions to our cultural toolkit.
Observational drawing is distinguished from other kinds of drawing by being specifically directed towards the depiction of observed objects and scenes as opposed to the depiction of imaginary or remembered subjects. It requires careful scrutiny and is an extremely time consuming skill both to learn and to practice, especially so in a world averse to all forms of delayed gratification.
Observational drawing is difficult not only because its techniques do not come easily but because in many ways it involves one of the most contradictory and absurd forms of observation known to exist. In short, it demands the deliberate restriction of observation in the following ways: maintaining a fixed point of view, closing one eye and occasionally squinting. None of these procedures is essential to observational drawing but without them, it becomes a significantly more challenging prospect both to learn and to perform.
It should be obvious that our perceptual skills are not perfect. But the reason they are not perfect has almost everything to do with our sensory capacities and very little to do with our brains. When we see a balloon "disappear" into the distance we do not suppose that it has actually vanished. All that has happened is that our retinas are no longer picking up sufficient stimuli for us to see the balloon. The further things recede, the less we see of them. The same goes for failing light. At a certain point the cones of the eye become insensitive and only the intensity-sensitive rods are stimulated. As the light continues to dim, even the rods become insensitive. Likewise, when we squint at a scene, in order to draw tonal values, we deliberately restrict the sensory stimulation to our intensity-sensitive rods.
So it turns out that observational drawing is actually a very restricted form of observation, a pseudo, quasi, or partial observation — a form of looking that we have to strive both to learn and to perform because in many important respects it goes against everything that perception has evolved to do, i.e., to see things as they actually are.