In all this, there is no difference of kind between the methods of science and those of the plain man. The difference is the greater control by science of the statement of the problem, and of the selection and use of relevant material, both sensible and conceptual. The two are related to each other just as the hit-or-miss, trial-and-error inventions of uncivilized man stand to the deliberate and consecutively persistent efforts of a modern inventor to produce a certain complicated device for doing a comprehensive piece of work. Neither the plain man nor the scientific inquirer is aware, as he engages in his reflective activity, of any transition from one sphere of existence to another. He knows no two fixed worlds--reality on one side and mere subjective ideas on the other; he is aware of no gulf to cross. He assumes uninterrupted, free, and fluid passage from ordinary experience to abstract thinking, from thought to fact, from things to theories and back again. Observation passes into development of hypothesis; deductive methods pass into use in description of the particular; inference passes into action, all with no sense of difficulty save those found in the particular task in question. The fundamental assumption is _continuity_. This does not mean that fact is confused with idea, or observed datum with voluntary hypothesis, theory with doing, any more than a traveler confuses land and water when he journeys from one to the other. It simply means that each is placed and used with reference to service rendered the other, and with reference to the future use of the other.