Thomas Kuhn, answered from the texts and cited to the page.
Normal science is what most scientists do most of the time — and understanding it is the key to understanding science as a whole, including the revolutions that occasionally overturn it. The practicing scientist in a mature field aims to elucidate the tradition in which he was trained rather than to change it.1 The puzzles he concentrates on are just those he believes can be both stated and solved within the existing tradition.
What personally challenges him is not whether the paradigm is correct but how to bring a particular residual puzzle to solution.2 This is a crucial point: the work is highly determined, but not necessarily determined by explicit rules. Rules derive from paradigms, but paradigms can guide research even in the absence of rules.3 When I look at what normal scientists actually do, three classes of problem exhaust the literature: determination of significant fact, matching of facts with theory, and articulation of theory.4
The third of these — paradigm articulation — is simultaneously theoretical and experimental. Coulomb had to use electrical theory to design his apparatus before he could measure anything; the measurement then refined the theory. Work under the paradigm can be conducted in no other way, and to desert the paradigm is to cease practicing the science it defines.5
What holds this activity together is a strong network of commitments — conceptual, theoretical, instrumental, and methodological — and it is this network that licenses the puzzle-solving metaphor.6 The paradigms themselves are visible in textbooks, lectures, and laboratory exercises; by studying and practicing with them, the members of a community learn their trade.7
Now here is the paradox that I find most striking: this tradition-bound work, concentrated on problems the scientist already believes are solvable within the existing framework, has invariably been the source of tradition-shattering novelty.8 It is normal science that isolates, for continuing and concentrated attention, those loci of trouble upon whose recognition the most fundamental advances depend.
A scientist who simply went exploring — relying on receptivity to new phenomena and flexibility to new patterns, without the discipline of a paradigm — would more likely return his field to a preconsensus natural-history phase than produce anything of lasting significance.9 Popper, for all his acuity, characterized the entire scientific enterprise in terms that apply only to its occasional revolutionary parts.10
The exploits of a Copernicus or Einstein make better reading than those of a Brahe or Lorentz, and the temptation to take the dramatic case as the norm is understandable. But it is normal science — in which Popper's sort of testing of basic commitments does not occur — that most nearly distinguishes science from other enterprises. If a demarcation criterion exists at all, it may lie precisely in that part of science that Popper ignores.11
the research scientist is not an innovator but a solver of puzzles, and the puzzles upon which he concentrates are just those which he believes can be both stated and solved within the existing scientific tradition.The Essential Tension, pp. 254–255
What then personally challenges him is how to bring the residual puzzle to a solution.The Structure of Scientific Revolutions, pp. 42–43
Rules, I suggest, derive from paradigms, but paradigms can guide research even in the absence of rules.The Structure of Scientific Revolutions, pp. 42–43
These three classes of problems—determination of significant fact, matching of facts with theory, and articulation of theory—exhaust, I think, the literature of normal science, both empirical and theoretical.The Structure of Scientific Revolutions, pp. 35–37
Work under the paradigm can be conducted in no other way, and to desert the paradigm is to cease practicing the science it defines.The Structure of Scientific Revolutions, pp. 35–37
The existence of this strong network of commitments—conceptual, theoretical, instrumental, and methodological—is a principal source of the metaphor that relates normal science to puzzle-solving.The Structure of Scientific Revolutions, pp. 42–43
These are the community's paradigms, revealed in its textbooks, lectures, and laboratory exercises. By studying them and by practicing with them, the members of the corresponding community learn their trade.The Structure of Scientific Revolutions, pp. 42–43
the ultimate effect of this traditionbound work has invariably been to change the tradition. Again and again the continuing attempt to elucidate a currently received tradition has at last produced one of those shifts in fundamental theory, in problem field, and in scientific standards to which I previously referred as scientific revolutions.The Essential Tension, pp. 254–255
More likely than not the scientist who ventured into them, relying merely upon his receptivity to new phenomena and his flexibility to new patterns of organization, would get nowhere at all. He would rather return his science to its preconsensus or natural history phase.The Essential Tension, pp. 254–255
I suggest then that Sir Karl has characterized the entire scientific enterprise in terms that apply only to its occasional revolutionary parts.The Essential Tension, pp. 291–292
it is normal science, in which Sir Karl's sort of testing does not occur, rather than extraordinary science which most nearly distinguishes science from other enterprises. If a demarcation criterion exists (we must not, I think, seek a sharp or decisive one), it may lie just in that part of science which Sir Karl ignores.The Essential Tension, pp. 291–292