"When should a theory be ranked as scientific? Is there a criterion for the scientific character of status of a theory?" ".....to distinguish between science and pseudo-science, knowing very well that science often errs and that pseudo-science may happen to stumble on the truth."
"There is of course the most widely accepted answer to the problem: the empirical method, which is essentially inductive, proceeding from observation to experiment."
"On the contrary...the problem is one of distinguishing between a genuinely empirical method and a non-empirical or even a pseudo-empirical method...which...although it appeals to observation and experiment, nevertheless does not come up to the scientific standards. (astrology versus astronomy)
"The theories of Marx, Freud and Adler...able to explain practically everything, whatever happens always confirmed it ....what precisely does it confirm? no more than that a case could be interpreted in the light of the theory.
"...precisely this fact -that they always fitted, that they were always confirmed -which in the eyes of their admirers constituted the strongest arguments in favor of these theories. It began to dawn on me that this apparent strength was in fact their weakness.
"With Einstein's theory the situation was strikingly different...the impressive thing was the risk involved in the prediction [of the amount by which light should be bent by gravity]. The theory was incompatible with certain possible results of observations -in fact with results which everybody before Einstein would have expected."
"Confirmations should count only if they are the result of risky predictions."
"Every good scientific theory is a prohibition: it forbids certain things to happen. the more a scientific theory forbids, the better it is."
"A theory which is not refutable by any conceivable event is non-scientific."
"Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifyability."
"Bold ideas, unjustified expectations and speculations constitute our only means for comprehending nature. Those of us who refuse to expose our own ideas to the risk of refutation are not the real participants in the game of science."

Kuhn's writings concern what one can call the sociology of science, how new concepts supplant older ones, specifically how this occurs in practice (perhaps inevitably), rather than how this ought to occur.
Two terms with special meanings in Kuhn's writings are "paradigm" and "revolution." By paradigm he means a conceptual framework, such a theory or set of theories, in terms of which the observed facts are interpreted and explained. What he means by a "revolution" is some major change in people's thinking, in which an old paradigm comes to be considered disproven or is otherwise discarded, being replaced by a new paradigm. Among Kuhn's major insights are:
(1) People tend to cling to their old paradigm, sometimes a little irrationally;
(2) Most science is an attempt (almost always a successful attempt) to fit new facts into the currently accepted paradigm; (and this can usually be done, even when the paradigm is very wrong!)
(3) Observations tend to be uninterpretable or otherwise meaningless unless they do fit into the accepted paradigm (however forced and "Procrustean" the fit!);
(4) An accepted paradigm is never discarded until a new one has been proposed. (A scientific revolution = paradigm-shift);
(5) Historical sections of textbooks tend to ignore or even hide the existence of those paradigms which preceded the currently accepted one, and to pretend that past discoveries had fit right into the current paradigm, or had even been motivated by it.

Some quotations from "The Structure of Scientific Revolutions" by Thomas Kuhn:

"The transition to a new paradigm is a scientific revolution." "Mopping-up operations are what engage most scientists throughout their careers. They constitute what I am here calling normal science." "Normal science does not aim at novelties, and when successful finds none."
".... (a) new theory implies a change in the rules governing the prior practice of normal science. ...a new theory....is seldom or never just an increment TO what is already known. Its assimilation requires the reconstruction of prior theory and the re-evaluation of prior fact."
"What Lavosier announced in his papers from 1777 on was not so much the discovery of oxygen as (it was) the oxygen theory of combustion. ...a reformulation of chemistry so vast that it is usually called the chemical revolution. ...the impossible suggestion that Priestly first discovered oxygen and Lavosier then invented it has its attractions."
".... once it has achieved the status of a paradigm, a scientific theory is declared invalid only if an alternative candidate is available to take its place. (In contrast to) ...the methodological stereotype of falsification by direct comparison with nature."
"Once a first paradigm through which to view nature has been found, there is no such thing as research in the absence of any paradigm. To reject one...without... substituting another is to reject science itself."
"The transition from a paradigm in crisis to a new one ...is far from a cumulative process....Rather, it is a reconstruction of the field from new fundamentals.
"Almost always the men who achieve these fundamental inventions of a new paradigm have been either very young or very new to the field..."
"Political revolutions aim to change political institutions in ways that these institutions themselves prohibit. Their success therefore necessitates the partial relinquishment of one set of institutions in favor of another, and in the interim, society is not fully governed by institutions."
"Science textbooks...refer only to that part of the work of past scientists that can easily be viewed as contributions to the statement and solutions of the text's paradigm problem. Partly by selection and partly by distortion, the scientists of earlier ages are implicitly represented as having worked on the same set of fixed problems and in accordance with the same set of fixed canons...(of)...the most recent revolution in science theory and method... No wonder... they have to be re-written after each scientific revolution. (so that science once again seems largely cumulative."

A post-modernist journal published at Duke (named "Social Text") fell for a practical joke by a physicist named Alan Sokal, back in the mid-90s. He wrote a parody article, making fun of the kinds of b.s. that they would like to believe, submitted it to their editor as if it were a real article, and they fell for the trick and published it in their journal. Sokal then announced that it was a joke, and proved the fraudulence of their whole field. This was part of what is now called "The Science Wars".