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Weaver's short essay is the historical starting point for the science of complexity, and the piece that named its central object. Writing in 1948, out of the wartime experience of operations research and the first electronic computers, Weaver sorts the problems science can pose into three kinds. Problems of simplicity involve a few variables (two or three) and were the triumph of the physical sciences from the seventeenth to the nineteenth century: the mechanics of a billiard ball, of planetary motion. Problems of disorganised complexity lie at the opposite extreme, involving enormous numbers of variables whose individual behaviour is erratic or unknown but whose aggregate can be described with great precision through the statistics of averages and probabilities, as in statistical mechanics or the actuarial treatment of a population. Between these two, and largely untouched by either the analytic methods of the first or the statistical methods of the second, lies the vast territory Weaver calls organised complexity: problems with a sizeable but moderate number of variables that are interrelated into an organic whole, so that they can be reduced neither to a simple formula nor to the law of averages. Most of the important questions in biology, medicine, economics, politics and the social sciences, he argues, are problems of this middle kind, and they are precisely the ones science had so far been least able to handle. The essay's second move is prescriptive: organised complexity will yield, Weaver predicts, to two developments born of the war, the electronic computer (which can hold and manipulate far more interacting variables than the unaided mind) and the interdisciplinary 'mixed team' whose members are drawn from many fields and work in genuine collaboration. For a corpus built around the complexity of organisations, Weaver supplies the founding vocabulary: the recognition that an organisation is neither a simple mechanism to be solved nor a mere aggregate to be averaged, but an organised complex whole, is the premise on which the later complex-adaptive-systems and ecological traditions (Holling, and in this corpus the resilience and normal-accident literatures) would be built. (Text drawn from the 1948 American Scientist essay as reproduced, with an editorial summary, in the 2004 E:CO Classical Papers reprint; the scanned original body was not machine-readable.)