Although the total stock of knowledge represents the everyday world in an integrated manner, differentiated according to zones of familiarity and remoteness, it leaves the totality of that world opaque. Put differently, the reality of everyday life always appears as a zone of lucidity behind which there is a background of darkness. As some zones of reality are illuminated, others are adumbrated. I cannot know everything there is to know about this reality. Even if, for instance, I am a seemingly all-powerful despot in my family, and know this, I cannot know all the factors that go into the continuing success of my despotism. I know that my orders are always obeyed, but I cannot be sure of all the steps and all the motives that lie between the issuance and the execution of my orders. There are always things that go on 'behind my back'. This is true a fortiori when social relationships more complex than those of the family are involved—and explains, incidentally, why despots are endemically nervous. My knowledge of everyday life has the quality of an instrument that cuts a path through a forest and, as it does so, projects a narrow cone of light on what lies just ahead and immediately around; on all sides of the path there continues to be darkness. This image pertains even more, of course, to the multiple realities in which everyday life is continually transcended. This latter statement can be paraphrased, poetically if not exhaustively, by saying that the reality of everyday life is overcast by the penumbras of our dreams.
My knowledge of everyday life is structured in terms of relevances. Some of these are determined by immediate pragmatic interests of mine, others by my general situation in society. It is irrelevant to me how my wife goes about cooking my favourite goulash as long as it turns out the way I like it. It is irrelevant to me that the stock of a company is falling, if I do not own such stock; or that Catholics are modernizing their doctrine, if I am an atheist; or that it is now possible to fly non-stop to Africa, if I do not want to go there. However, my relevance structures intersect with the relevance structures of others at many points, as a result of which we have 'interesting' things to say to each other. An important element of my knowledge of everyday life is the knowledge of the relevance structures of others. Thus 'I know better' than to tell my doctor about my investment problems, my lawyer about my ulcer pains, or my accountant about my quest for religious truth. The basic relevance structures referring to everyday life are presented to me ready-made by the social stock of knowledge itself. I know that 'woman talk' is irrelevant to me as a man, that 'idle speculation' is irrelevant to me as a man of action, and so forth. Finally, the social stock of knowledge as a whole has its own relevance structure. Thus, in terms of the stock of knowledge objectivated in American society, it is irrelevant to study the movemements of the stars to predict the stock market, but it is relevant to study an individual's slips of the tongue to find out about his sex life, and so on. Conversely, in other societies, astrology may be highly relevant for economics, speech analysis quite irrelevant for erotic curiosity, and so on.
One final point should be made here about the social distribution of knowledge. I encounter knowledge in everyday life as socially distributed, that is, as possessed differently by different individuals and types of individuals. I do not share my knowledge equally with all my fellowmen, and there may be some knowledge that I share with no one. I share my professional expertise with colleagues, but not with my family, and I may share with nobody my knowledge of how to cheat at cards. The social distribution of knowledge of certain elements of everyday reality can become highly complex and even confusing to the outsider. I not only do not possess the knowledge supposedly required to cure me of a physical ailment, I may even lack the knowledge of which one of a bewildering variety of medical specialists claims jurisdiction over what ails me. In such cases, I require not only the advice of experts, but the prior advice of experts on experts. The social distribution of knowledge thus begins with the simple fact that I do not know everything known to my fellowmen, and vice versa, and culminates in exceedingly complex and esoteric systems of expertise. Knowledge of how the socially available stock of knowledge is distributed, at least in outline, is an important element of that same stock of knowledge. In everyday life I know, at least roughly, what I can hide from whom, whom I can turn to for information on what I do not know, and generally which types of individuals may be expected to have which types of knowledge.
(from ch. 2, "Language and Knowledge in Everyday Life")