The reason tacit knowledge exists and is essential is because the brain is immensely complex, and most of what it does is outside consciousness.<p>To take just one well-studied example, in vision photons hit visual receptors in the retina, and then go through the optic nerve which is a complex computer that analyses the signals in stages, like picking out points, then lines, then shapes, and so on, and then it is sent to several different visual areas of the brain, each of which pulls out different features, like motion, colors, 3d shapes, faces, and emotional expressions, and then finally it arrives in consciousness. Something similar is true for every other sensation, and how they are integrated to give us our perception and understanding of the world.<p>As a consequence, children from birth learn all sorts of things without explicit instruction and often even much focal thought, include the great majority of the rules of language itself.<p>All of this is subjectively experienced as a sort of feel or intuition. It is described by philosophers such as Michael Polanyi and various phenomenologists. I studied with one of them, Eugene Gendlin, who developed a method called experiential focusing for helping people make use of their tacit knowledge (he called it felt meaning). You might want to take a look at his book Focusing.<p>One more point. The idea that all knowledge can be clearly explicated is an example of the denial of human finitudes that is common in much of Western philosophy, but that are affirmed by other schools Western philosophy, such as pragmatism, existential-phenomenology, and post-Wittgensteinian analytic philosophy.