Alan Turing

Abstract It is suggested that a system of chemical substances, called morphogens, reacting together and diffusing through a tissue, is adequate to account for the main phenomena of morphogenesis. Such a system, although it may originally be quite homogeneous, may later develop a pattern or structure due to an instability of the homogeneous equilibrium, which is triggered off by random disturbances. Such reaction-diffusion systems are considered in some detail in the case of an isolated ring of cells, a mathematically convenient, though biologically unusual system. The investigation is chiefly concerned with the onset of instability. It is found that there are six essentially different forms which this may take. In the most interesting form stationary waves appear on the ring. It is suggested that this might account, for instance, for the tentacle patterns on Hydra and for whorled leaves. A system of reactions and diffusion on a sphere is also considered. Such a system appears to account for gastrulation. Another reaction system in two dimensions gives rise to patterns reminiscent of dappling. It is also suggested that stationary waves in two dimensions could account for the phenomena of phyllotaxis. The purpose of this paper is to discuss a possible mechanism by which the genes of a zygote may determine the anatomical structure of the resulting organism. The theory does not make any new hypotheses; it merely suggests that certain well-known physical laws are sufficient to account for many of the facts. The full understanding of the paper requires a good knowledge of mathematics, some biology, and some elementary chemistry. Since readers cannot be expected to be experts in all of these subjects, a number of elementary facts are explained, which can be found in text-books, but whose omission would make the paper difficult reading.

Abstract Together with ‘On Computable Numbers’, ‘Computing Machinery and Intelligence’ forms Turing’s best-known work. This elegant and sometimes amusing essay was originally published in 1950 in the leading philosophy journal Mind. Turing’s friend Robin Gandy (like Turing a mathematical logician) said that ‘Computing Machinery and Intelligence’… was intended not so much as a penetrating contribution to philosophy but as propaganda. Turing thought the time had come for philosophers and mathematicians and scientists to take seriously the fact that computers were not merely calculating engines but were capable of behaviour which must be accounted as intelligent; he sought to persuade people that this was so. He wrote this paper—unlike his mathematical papers—quickly and with enjoyment. I can remember him reading aloud to me some of the passages— always with a smile, sometimes with a giggle. The quality and originality of ‘Computing Machinery and Intelligence’ have earned it a place among the classics of philosophy of mind. ‘Computing Machinery and Intelligence’ contains Turing’s principal exposition of the famous ‘imitation game’ or Turing test. The test first appeared, in a restricted form, in the closing paragraphs of ‘Intelligent Machinery’ (Chapter 10). Chapters 13 and 14, dating from 1951 and 1952 respectively, contain further discussion and amplification; unpublished until 1999, this important additional material throws new light on how the Turing test is to be understood. The imitation game involves three participants: a computer, a human interrogator, and a human ‘foil’. The interrogator attempts to determine, by asking questions of the other two participants, which of them is the computer. All communication is via keyboard and screen, or an equivalent arrangement (Turing suggested a teleprinter link). The interrogator may ask questions as penetrating and wide-ranging as he or she likes, and the computer is permitted to do everything possible to force a wrong identification. (So the computer might answer ‘No’ in response to ‘Are you a computer?’ and might follow a request to multiply one large number by another with a long pause and a plausibly incorrect answer.) The foil must help the interrogator to make a correct identification.