OSE volume 1 page 57:
the need to distinguish between two different elements in man's environment--his natural environment and his social environment. This is a distinction which is difficult to make and to grasp, as can be inferred from the fact that even now it is not clearly established in our minds ... Most of us, it seems, have a strong inclination to accept the peculiarities of our social environment as if they were 'natural'.
It is one of the characteristics of the magical attitude of a primitive tribal or 'closed' society that it lives in a charmed circle of unchanging taboos, of laws and customs which are felt to be as inevitable as the rising of the sun, or the cycle of the seasons, or similar obvious regularities of nature. And it is only after this magical 'closed society' has actually broken down that a theoretical understanding of the difference between 'nature' and 'society' can develop.
OSE volume 2 pages 89-90:
I am developing a view to which I subscribe myself ... the problem of the so-called rules of exogamy, i.e. the problem of explaining the wide distribution, among the most diverse cultures, of marriage laws apparently designed to prevent inbreeding ... [J.S.] Mill [and others] ... would try to explain these rules by an appeal to 'human nature' ... and something like this would also be the naive or popular explanation ... however, one could ask whether it is not the other way round ... whether the apparent instinct is not rather a product of education, the effect rather than the cause of the social rules and traditions demanding exogamy and forbidding incest ... [in this case] it would be difficult to determine which of the two theories is the correct one, the explanation of the traditional social rules by instinct or the explanation of an apparent instinct by traditional social rules ... [example where Karl Popper says experiments have shown that aversion to snakes, which was commonly thought to be instinct, is actually learned] ... This example should be taken as a warning. We are faced here with an aversion which is apparently universal, even beyond the human race ... The universal occurrence of a certain behavior is not a decisive argument in favour of its instinctive character, or of its being rooted in 'human nature'.