Scientism is the ideology of science. The term scientism generally points to the cosmetic application of science in unwarranted situations not amenable to application of the scientific method or similar scientific standards.
In philosophy of science, the term scientism frequently implies a critique of the more extreme expressions of logical positivism and has been used by social scientists such as Friedrich Hayek, philosophers of science such as Karl Popper, and philosophers such as Hilary Putnam and Tzvetan Todorov to describe (for example) the dogmatic endorsement of scientific methodology and the reduction of all knowledge to only that which is measured or confirmatory.
More generally, scientism is often interpreted as science applied "in excess". The term scientism has two senses:
It is also sometimes used to describe universal applicability of the scientific method and approach, and the view that empirical science constitutes the most authoritative worldview or the most valuable part of human learning--to the complete exclusion of other viewpoints, such as historical, philosophical, economic or cultural worldviews. It has been defined as "the view that the characteristic inductive methods of the natural sciences are the only source of genuine factual knowledge and, in particular, that they alone can yield true knowledge about man and society". The term scientism is also used by historians, philosophers, and cultural critics to highlight the possible dangers of lapses towards excessive reductionism in all fields of human knowledge.
For social theorists in the tradition of Max Weber, such as Jürgen Habermas and Max Horkheimer, the concept of scientism relates significantly to the philosophy of positivism, but also to the cultural rationalization for modern Western civilization. British writer and feminist thinker Sara Maitland has called scientism a "myth as pernicious as any sort of fundamentalism."
Reviewing the references to scientism in the works of contemporary scholars, Gregory R. Peterson detects two main broad themes:
The term scientism was popularized by the Nobel Prize winner F.A. Hayek, who defined it as the "slavish imitation of the method and language of Science". Karl Popper defines scientism as 'the aping of what is widely mistaken for the method of science'. 
Mikael Stenmark proposes the expression scientific expansionism as a synonym of scientism. In the Encyclopedia of science and religion, he writes that, while the doctrines that are described as scientism have many possible forms and varying degrees of ambition, they share the idea that the boundaries of science (that is, typically the natural sciences) could and should be expanded so that something that has not been previously considered as a subject pertinent to science can now be understood as part of science (usually with science becoming the sole or the main arbiter regarding this area or dimension).
According to Stenmark, the strongest form of scientism states that science has no boundaries and that all human problems and all aspects of human endeavor, with due time, will be dealt with and solved by science alone. This idea has also been called the Myth of Progress.
E. F. Schumacher, in his A Guide for the Perplexed, criticized scientism as an impoverished world view confined solely to what can be counted, measured and weighed. "The architects of the modern worldview, notably Galileo and Descartes, assumed that those things that could be weighed, measured, and counted were more true than those that could not be quantified. If it couldn't be counted, in other words, it didn't count."
Intellectual historian T.J. Jackson Lears argues there has been a recent reemergence of "nineteenth-century positivist faith that a reified 'science' has discovered (or is about to discover) all the important truths about human life. Precise measurement and rigorous calculation, in this view, are the basis for finally settling enduring metaphysical and moral controversies." Lears specifically identifies Harvard psychologist Steven Pinker's work as falling in this category. Philosophers John N. Gray and Thomas Nagel have leveled similar criticisms against popular works by moral psychologist Jonathan Haidt, neuroscientist Sam Harris, and writer Malcolm Gladwell.
Several scholars use the term to describe the work of vocal critics of religion-as-such. Individuals associated with New Atheism have garnered this label from both religious and non-religious scholars. Theologian John Haught argues that Daniel Dennett and other new atheists subscribe to a belief system of scientific naturalism, which holds the central dogma that "only nature, including humans and our creations, is real: that God does not exist; and that science alone can give us complete and reliable knowledge of reality." Haught argues that this belief system is self-refuting since it requires its adherents to assent to beliefs that violate its own stated requirements for knowledge. Christian Philosopher Peter Williams argues that it is only by conflating science with scientism that new atheists feel qualified to "pontificate on metaphysical issues." Philosopher Daniel Dennett responded to religious criticism of his book Breaking the Spell: Religion as a Natural Phenomenon by saying that accusations of scientism "[are] an all-purpose, wild-card smear... When someone puts forward a scientific theory that [religious critics] really don't like, they just try to discredit it as 'scientism'. But when it comes to facts, and explanations of facts, science is the only game in town".
Non-religious scholars have also linked New Atheist thought with scientism. Atheist philosopher Thomas Nagel argues that neuroscientist Sam Harris conflates all empirical knowledge with that of scientific knowledge. Marxist literary critic Terry Eagleton argues that Christopher Hitchens possesses an "old-fashioned scientistic notion of what counts as evidence" that reduces knowledge to what can and cannot be proven by scientific procedure. Agnostic philosopher Anthony Kenny has also criticized New Atheist philosopher Alexander Rosenberg's The Atheist's Guide to Reality for resurrecting a self-refuting epistemology of logical positivism and reducing all knowledge of the universe to the discipline of physics.
Michael Shermer, founder of The Skeptics Society, draws a parallel between scientism and traditional religious movements, pointing to the cult of personality that develops around some scientists in the public eye. He defines scientism as a worldview that encompasses natural explanations, eschews supernatural and paranormal speculations, and embraces empiricism and reason.
Gregory R. Peterson writes that "for many theologians and philosophers, scientism is among the greatest of intellectual sins".
In his essay Against Method, Paul Feyerabend characterizes science as "an essentially anarchic enterprise" and argues emphatically that science merits no exclusive monopoly over "dealing in knowledge" and that scientists have never operated within a distinct and narrowly self-defined tradition. He depicts the process of contemporary scientific education as a mild form of indoctrination, aimed at "making the history of science duller, simpler, more uniform, more 'objective' and more easily accessible to treatment by strict and unchanging rules."
[S]cience can stand on its own feet and does not need any help from rationalists, secular humanists, Marxists and similar religious movements; and... non-scientific cultures, procedures and assumptions can also stand on their own feet and should be allowed to do so... Science must be protected from ideologies; and societies, especially democratic societies, must be protected from science... In a democracy scientific institutions, research programmes, and suggestions must therefore be subjected to public control, there must be a separation of state and science just as there is a separation between state and religious institutions, and science should be taught as one view among many and not as the one and only road to truth and reality.-- Feyerabend, Against Method, p. viii
Thomas M. Lessl argues that religious themes persist in what he calls scientism, the public rhetoric of science. There are two methodologies that illustrate this idea of scientism. One is the epistemological approach, the assumption that the scientific method trumps other ways of knowing and the ontological approach, that the rational mind reflects the world and both operate in knowable ways. According to Lessl, the ontological approach is an attempt to "resolve the conflict between rationalism and skepticism". Lessl also argues that without scientism, there would not be a scientific culture.
Philosopher of religion Keith Ward has said scientism is philosophically inconsistent or even self-refuting, as the truth of the statements "no statements are true unless they can be proven scientifically (or logically)" or "no statements are true unless they can be shown empirically to be true" cannot themselves be proven scientifically, logically, or empirically.
In the introduction to his collected oeuvre on the sociology of religion, Max Weber asks why "the scientific, the artistic, the political, or the economic development [elsewhere]... did not enter upon that path of rationalization which is peculiar to the Occident?" According to the distinguished German social theorist, Jürgen Habermas, "For Weber, the intrinsic (that is, not merely contingent) relationship between modernity and what he called 'Occidental rationalism' was still self-evident." Weber described a process of rationalisation, disenchantment and the "disintegration of religious world views" that resulted in modern secular societies and capitalism.
"Modernization" was introduced as a technical term only in the 1950s. It is the mark of a theoretical approach that takes up Weber's problem but elaborates it with the tools of social-scientific functionalism... The theory of modernization performs two abstractions on Weber's concept of "modernity". It dissociates "modernity" from its modern European origins and stylizes it into a spatio-temporally neutral model for processes of social development in general. Furthermore, it breaks the internal connections between modernity and the historical context of Western rationalism, so that processes of modernization... [are] no longer burdened with the idea of a completion of modernity, that is to say, of a goal state after which "postmodern" developments would have to set in. ...Indeed it is precisely modernization research that has contributed to the currency of the expression "postmodern" even among social scientists.
Habermas is critical of pure instrumental rationality, arguing that the "Social Life-World" is better suited to literary expression, the former being "intersubjectively accessible experiences" that can be generalized in a formal language, while the latter "must generate an intersubjectivity of mutual understanding in each concrete case":
The world with which literature deals is the world in which human beings are born and live and finally die; the world in which they love and hate, in which they experience triumph and humiliation, hope and despair; the world of sufferings and enjoyments, of madness and common sense, of silliness, cunning and wisdom; the world of social pressures and individual impulses, of reason against passion, of instincts and conventions, of shared language and unsharable feelings and sensations...
There are criticisms of orthodox, 19th Century scientism and I intend to continue with this enterprise.
Scientism: Pejorative term for the belief that the methods of natural science, or the categories and things recognized in natural science, form the only proper elements in any philosophical or other inquiry.
His essay, a defense of "scientism," is a long exercise in assimilating humanistic inquiries into scientific ones. By the time Pinker is finished, the humanities are the handmaiden of the sciences, and dependent upon the sciences for their advance and even their survival.
...scientism is a revival of the nineteenth-century positivist faith that a reified "science" has discovered (or is about to discover) all the important truths about human life. Precise measurement and rigorous calculation, in this view, are the basis for finally settling enduring metaphysical and moral controversies--explaining consciousness and choice, replacing ambiguity with certainty.
The term scientism is ordinarily used with pejorative intent.
The term 'scientism' is sometimes used in a pejorative sense
Scientism... a term of abuse since Friedrich Hayek first popularized it in the 1940s..
These theories show the continuing appeal of scientism--the modern belief that scientific inquiry can enable us to resolve conflicts and dilemmas in contexts where traditional sources of wisdom and practical knowledge seem to have failed.
... the mix of moralism and scientism is an ever-winning formula, as Gladwell's career demonstrates.
Harris urges that we use scientific knowledge about humans to discover what will maximize their well-being, and thereby to discover the right way to live. This is an instrumental use of science, starting out from his basic moral premise.
He says that the discovery of moral truth depends on science, but this turns out to be misleading, because he includes under "science" all empirical knowledge of what the world is like...Harris urges that we use scientific knowledge about humans to discover what will maximize their well-being, and thereby to discover the right way to live.
The main tenets of this philosophy are bracingly summed up in a series of questions and answers: Is there a God? No. What is the nature of reality? What physics says it is.
the best way to understand the charge of scientism is as a kind of logical fallacy involving improper usage of science or scientific claims.