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Rule IV. There is need of a method for finding out the truth. Rule V. Method consists entirely in the order and disposition of the objects toward which our mental vision must be directed if we would find out any truth. We shall comply with it exactly if we reduce involved and obscure propositions step be step to those that are s- pler, and then starting with the intuitive apprehension of all those that are absolutely simple, attempt to ascend to the knowledge of all others by precisely similar steps. Rene Descartes, Rules for the Direction of Mind Perhaps he would sooner satisfy himself by resolving light into colours as far as may be done by Art, and then by examining the properties of those colours apart, and afterwards by trying the effects of reconjoyning two or more or all of those, and lastly by separating them again to examine what changes that reconjunction had wrought in them. This will prove a tedious and difficult task to do it as it ought to be done but I could not be satisfied till I had gone through it. From Newton s letter, quoted in The Life of Isaac Newton by Richard Westfall. Cambridge University Press, 1993.
