Whitehead does not elaborate on the great nineteenth century discovery of the method of invention. But it is, quite simply, the technique of beginning at the end of any operation whatever, and of working backwards from that point to the beginning. It is the method inherent in the Gutenberg technique of homogeneous segmentation, but not until the nineteenth century was the method extended from production to consumption. Planned production means that the total process must be worked out in exact stages, backwards, like a detective story. In the first great age of mass production of commodities, and of literature as a commodity for the market, it became necessary to study the consumer's experience. In a word it became necessary to examine the effect of art and literature before producing anything at all. This is the literal entrance to the world of myth.