Why does Fayerabend see observation terms, scientific method and fixed standards as fetishes of rationalism?
Feyerabend makes a detailed demolition of the so called "received view" of philosophy of science (i.e.Logical Empiricism and Popper) and its attempts to discover rules of scientific method.
In his book Against Method (1975) he uses historical case studies (i.e. the transition from older scientific theories, like Aristotle's theory of motion, to the scientific revolution of Galileo) to argue that there are no universal and exceptionless methodological rules governing the progress of science or the growth of knowledge.
According to F, the history of science is so complex that if we aim at a general methodology that is applicable in every historical context and to all objects of study, the only “rule” that is applicable will be the trivial one: “anything goes”. The phenomenon of incommensurability between different "points of view" renders the standards which the “rationalists” (like Popper) use for comparing competing theories inapplicable.
The critics of F's book accused him of beeing an “irrationalist”.
See: Paul Feyerabend, in Stanford Encyclopedia of Philosophy.