Logical positivism arose between the world wars primarily among a group of Austrian intellectuals (philosophers, scientists, and mathematicians) known as the Vienna Circle, which included philosophers Moritz Schlick (1882–1936), Otto Neurath (1882–1945), and Rudolf Carnap (1891–1970). The most general and fundamental thesis of the logical positivists was a strict and new interpretation of empiricism, which included a clear and sometimes virulent rejection and repudiation of speculative metaphysics. In addition, they held common theses regarding science, most notably linguistic theses based on the analytic–synthetic distinction and verifiability theory of meaning.

EMPIRICISM

In its most general sense, empiricism defines a broad epistemological view in which knowledge is attained, affirmed, and even defined by sensory perception. In this broad sense, empirical views can be found throughout the history of philosophy. The term can also be used to refer to more localized and specified epistemological views, such as those of the logical positivists or the British empiricists. This epistemological view is typically contrasted with the rationalist view of knowledge. Rationalists hold that knowledge is attained and affirmed not by sensory perception but by the internal operations and content of the human mind, arguing that the human mind contains at birth certain innate ideas: beliefs, statements, or principles that are empirically unprovable but generally accepted as intuitively true. Examples from noted rationalists of innate ideas include the existence of God, the existence of one’s self, and the noncontradiction principle, as well as other basic logical and mathematical truths. Rationalists also typically hold that sensory perception does not provide real knowledge because sensory perception is often susceptible to illusion and error, and knowledge is classically understood as both true and lasting. However, much of what we perceive is often not true (e.g., the “bent” pencil in the glass of water) and often not lasting (e.g., even the oldest of mountains we see will one day erode and be no more; even the stars in the sky will one day blink out). Real knowledge (e.g., logical and mathematical truths), according to rationalists, is true and will always be true. Empiricists tend to find this view of knowledge and these “truths” of rationalists somewhat empty and circular. Sure, one may rationally intuit that one and one is two, but until one affirms that this mental function has a relationship to the physical world, it is purely formal and of no use-value. Sure, one may rationally intuit one’s own existence, but how does this aid someone in living in this world, or even understanding the world beyond one’s own mind?

The logical positivists were especially influenced by the school of empiricism known as British Empiricism. This was a school of philosophy from the 17th and 18th centuries. The primary figures in this school were from the British Isles, hence the name: John Locke (Britain, 1632–1704), George Berkeley (Ireland, 1685–1753), and David Hume (Scotland, 1711–1776). These philosophers were only very loosely connected under the heading of British Empiricism. In detail, there were many differences in their thinking. However, most generally it can be said that they all agreed that the primary (if not sole) source of knowledge was sensory perception. They all denied the concept of innate ideas. John Locke famously expressed this rejection by declaring the mind a blank slate or tabula rasa at birth; all knowledge would then be imprinted on the mind through sensory experience. Moreover, they tended to see knowledge as reducible to “ideas” or sense data. The word idea here is a technical term in the school of British Empiricism. It refers to discrete and specifiable contents of the mind. That is, the concept of, say, “table” is an idea. The memory of a specific table is also an idea. So are the constituent sensory qualities of that table: its color, its shape, its hardness, and the sound produced when knocking on it. The collection of all these ideas comprises the knowledge contained in our minds. As there are no innate ideas, these ideas can all be traced back to sense perceptions, which enter our minds as discrete sense data, that is, discrete bits of sensory information. According to John Locke, we generate general and abstract ideas like “table” by comparing various specific tables seen, felt, and so forth, and then abstracting the similar qualities among them.

The British Empiricists interestingly seemed to become more strictly empirical over time. John Locke was willing to accept the existence of some empirically controversial ideas, such as God and material substance (the matter that underlies our sense perceptions). George Berkeley argued against the existence of physical matter. These arguments of Berkeley have been infamously lampooned, but they raise an important question regarding a strict adherence to empirical principles. Strictly speaking, all we can “know” are the direct sensory impressions we experience. We do not have direct experience of a substrate or physical matter underlying those sense impressions. Consider the movie The Matrix (Wachowski & Wachowski, 1999). Part of the story of this movie is that people are living an illusion. Presumably, they live lives similar to ours with similar experiences and similar vibrancy of sense impressions. In other words, their world seems just as real to them as ours does to us. However, it isn’t. The reality is that people are being kept in pods as sort of batteries for the computers really in charge of the world. The world of sense impressions they experience is in fact a kind of virtual reality piped directly into their brains/minds. So there is no substrate or physical matter underlying these impressions. There is no way to prove that we are not in a similar position, that is, there is no way of proving that there is a material substrate underlying our experience of the “physical” world. This question raises certain metaphysical problems for any strict empiricist. Berkeley simply accepted nonexistence of the physical world and maintained that reality was “ideal,” which means composed of nonphysical or immaterial entities, such as ideas, souls, and God. And rather than from a physical substrate, the sense impressions we receive come directly from God. This may seem a rather farfetched view, but there is a spiritual beauty to it. Rather than God having created the universe at one point in the far past, God is continually creating the universe by providing the sense impressions that comprise our world. There may also be a theological ulterior motive to Berkeley’s position. Throughout the modern era and the Scientific Revolution, modern science (the subject of which was primarily matter) was in some ways supplanting God in providing explanations for phenomena. To Berkeley, an Anglican bishop, this may have seemed like an eclipsing of God. By removing matter from the equation and asserting God as the source of all our sense impressions, Berkeley perhaps meant to place God at the center of our conceptual universe again. Now, Berkeley did not deny the knowledge, authority, laws, and advances of science. He merely maintained that the subject of science was not matter but the ideas that come from God. The laws science discovers are real laws. However, they are not laws about matter but about the ideas provided by God. Those ideas manifest the structure God gives to our ideal world. So, Berkeley was not really denying reality or the reality of what we know, as some mistakenly assume. He was merely denying that our knowledge was knowledge of, or based on, physical matter. In fact, this knowledge comes directly from God. It is difficult to get more real than that, assuming of course that God is real. Being the last of these British empiricists, David Hume was arguably the most empirically strict. He recognized the difficulty of proving the existence of matter and remained somewhat agnostic on that question. Regarding the idea of God, Hume was an uncompromising atheist, as we have no direct empirical evidence for God’s existence. He even questioned the epistemological authority of causality as a force for which we have no direct empirical experience. We address that issue in some detail in Chapter 8.

British Empiricism is typically contrasted to the contemporaneous school of philosophy known as Continental Rationalism. The “continental” in the name of this school refers to the fact that this school of philosophy originated in continental Europe as opposed to the British Isles. The three primary philosophers in this school are René Descartes (France, 1596–1650), Baruch Spinoza (The Netherlands, 1632–1677), and Gottfried Leibniz (Germany, 1646–1716). As noted earlier, rationalists tend to hold that real knowledge is a pure product of the mind, based on innate ideas and the mental functions of logic and mathematics. This school of rationalism has been especially criticized for indulgence of metaphysical speculation. From an empiricist point of view, without being grounded in sensory perception, the musings of rationalists have nothing to hold them down, allowing rationalists to posit entities (e.g., Leibniz’s monads) that seem to spring directly from the mind with no existential substantiation beyond the mere possibility of their existence. Immanuel Kant (1724–1804) attempted to reconcile the two schools, recognizing the strengths and weaknesses of the two positions, noting that, “though all our knowledge begins with experience, it does not follow that it all arises out of experience” (Kant, 1781/1965, p. 41) and “Thoughts without content are empty, intuitions without concepts are blind” (Kant, 1781/1965, p. 93). Kant argued that although there may not be innate ideas as the Continental Rationalists understood them, there were certain a priori innate structures in the human mind that bring comprehensible form to direct sensory experience. Thus, without content (sensory experience) we have an empty mind with a basic structure ready to accept sense impressions, like a computer with no data input, but no actual knowledge. Without these basic, innate structures, sensory experience is incomprehensible, without form, like the “blooming, buzzing confusion” of a newborn baby’s experience of the world or like inputting data into a computer lacking an operating system (James, 2007, p. 488). The degree of Kant’s success in reconciling these schools has been a matter of philosophical debate for the past couple hundred years.

Just as the logical positivists were influenced by empiricism (especially British Empiricism), they also defined themselves against the school of rationalism and the speculative metaphysics to which it gave rise. They expressed a commitment to knowledge based in sensory experience and observation. This attitude also reflects the admiration for science they typically espoused. They seemed to study and critique science out of a love for it and admiration for scientists of their day, like Alfred Einstein. Even more than the metaphysical flights of the Continental Rationalists they reviled the obscurantist (in their view) metaphysical writings of Gottfried Hegel (1770–1831), the most influential post-Kantian German philosopher in the 19th century. They found his writing purposefully obscure, contrary to basic logic, and replete with meaningless metaphysical speculation, regarding such mysterious concepts as the Absolute.

POSITIVISM

Another important influence on the logical positivists was the French philosopher Auguste Comte (1798–1857), whose philosophy was known as positivism, the obvious inspiration for half of the term logical positivism. Comte was also a sociologist, in fact one of the founders of that science. The idea that he is most known for, and which was likely most influential on the logical positivists, was his law of three phases. He proposed that societies evolve through three identifiable and progressive stages. The first stage, the theological, is one in which a society refers all explanations of phenomena to God or some similar religious concept. The second stage, the metaphysical, is marked by a turn to rational thinking but without a foundation in observation and empirical research. The third stage, the scientific, is the highest stage and marked by a commitment to rational and empirical investigation. This third stage takes on a utopian character in his writings. It is through scientific thinking (not theological or metaphysical) that the human race and human society will be perfected and human goods will be fulfilled. It would be the logical positivists’ contention, then, that we are finally (and fortunately) in this final stage of development.

CENTRAL IDEAS

Logical positivists’ views about science and knowledge are based on a general theory of language formulated around two principles: the analytic–synthetic distinction and the verifiability theory of knowledge. According to the philosophical view, these two principles work together to form a coherent and comprehensive theory of knowledge, providing a clear and definite standard for determining what we know.

The Analytic–Synthetic Distinction

According to the analytic–synthetic distinction, statements can be divided into those that are analytic and those that are synthetic. Analytic statements are true or false based merely on their meaning, regardless of how the world really is. One example of an analytic statement is “All bachelors are unmarried.” This statement is true because the quality of being unmarried is included within the meaning or definition of “bachelor.” Thus, it is also said that analytic statements are true by definition. Another way to describe analytic statements is to say that the predicate (in this case “being unmarried”) is included in the subject (“bachelors”). Many philosophers (including the logical positivists) believe that mathematical statements are analytic statements. For example, to say that one and one is two is to say, analytically, that the predicate “two” is included in the subject “one and one.” This is what makes mathematics so certain and absolute. The truth of these statements is dependent merely on the meaning of the terms involved and the logical relationship asserted. Synthetic statements, on the other hand, are true or false based on both meaning and the states of the world. An example of a synthetic statement would be “All bachelors are short.” Synthetic statements are “synthetic” in the sense of bringing disparate elements together. The concept of “shortness” is not included in the concept of “bachelor.” Thus, the two are separate, distinct, and brought together (“synthesized”) in the aforementioned statement. The truth of the statement is dependent on the meaning of “bachelors,” the meaning of “short,” the logical relationship asserted between the two, and a state of the world. If the world is such that each and every bachelor is short, then the statement is true. If even one bachelor is not short, then the statement is false.

Logical positivists hold that only synthetic statements can make meaningful claims about the world. That is, there are epistemological implications to these two types of statements. Analytic statements are typically accepted as expressing what is called a priori knowledge. “A priori” refers to knowledge that is attained without or prior to sensory experience. This is the type of knowledge that rationalists tend to accept and affirm as real knowledge, because it is necessarily true. This necessary truth is expressed through analytic statements due to the predicate being part of the subject. However, that aspect of analytic statements and a priori knowledge demonstrates the limitation of analytic truths. Such knowledge is somewhat incestuous. Analytic statements only tell us about the ways in which we use symbols, such as “bachelor” and “unmarried.” They do not give us information about the world itself. Their truth, meaning, and affirmation are completely contained within the mind. Synthetic statements, on the other hand, assert facts about the world outside the mind. In order to truthfully assert that “all bachelors are short,” we must look to the world and attain information through sensory experience. This type of knowledge is called a posteriori knowledge: knowledge attained due to or after sensory experience. Being dependent on sensory experience, synthetic statements and a posteriori knowledge do not have the certainty of analytic statements and a priori knowledge, but they are, according to logical positivists, more meaningful. Thus, according to logical positivists, analytic statements, having no concrete information about the world, are empty; while synthetic statements are filled with content and are meaningful in that sense.

The Verifiability Theory of Knowledge

The verifiability theory of meaning holds that the meaning of a (synthetic) statement is determined, and even defined, by its verification. For example, the meaning of the statement, “Water boils at 100°C at sea level” is dependent on the experiments that could be done to verify this claim. The only real verification for logical positivism is empirical observation. If a statement cannot be verified through empirical observation, it has no meaning. Although one does not have to in fact test a claim to make it meaningful, there must simply exist, at least in principle, a means of verifying its truth. Consider the hypothetical case of Planet X. Planet X is a planet in our solar system that has not yet been discovered. The reason it has not been discovered is that it shares its orbit with Earth’s orbit in such a way that the sun is always between the Earth and Planet X. The statement “Planet X exists” is verifiable in principle but not, at the present time, in practice. If we had a powerful enough spaceship, we could fly around the sun and verify this claim. So the statement “Planet X exists” is a meaningful statement, even though we cannot verify it (or refute it) at this time.

The logical positivists used the verifiability theory of meaning to criticize, even attack, much philosophy of their day and of the past. Their aforementioned disagreement with speculative metaphysics was often framed around this principle. Claims about substances beyond sensory experience or metaphysical concepts, such as forms, monads, and immaterial minds, were judged, under the verifiability theory of meaning, meaningless. Logical positivists were also highly critical of religious claims and much ethical and aesthetic discourse. All traffic in claims about forces and entities that are empirically unverifiable: claims about God, souls, the afterlife, good, evil, and beauty. They were also critical of psychology and not interested in psychological or historical analyses of science. The psychological state of a scientist when making a discovery was, for the positivists, irrelevant. A discovery is a discovery regardless of a scientist’s mental state. The history of science was likewise irrelevant to their studies. For historians, such a story might be of interest, but in trying to understand what science itself is, the history behind it is not relevant. For logical positivists science existed as a pure study, not affected by the whims of speculative metaphysics, religion, history, or psychology. Science was reducible to simply logic and empirical observation.

The Rise and Fall of Logical Positivism

The logical positivists, in their day, were seen as radical and revolutionary. They were asserting a fresh, new, and empirically strict approach to philosophy. These days they are often seen as old and stuffy. That which was new and revolutionary quickly becomes the status quo, the new institution. By the 1950s, cracks were beginning to emerge in their framework. Even in the 1940s the movement underwent changes, even a change of name as it came to call itself logical empiricism. In the literature of philosophy of science, the terms logical positivism and logical empiricism are not treated in consistent manners. Some writers will use the terms interchangeably. Some will use one or another to refer to the whole movement. And some will use the former to refer to the earlier theorists (roughly writing before World War II [WWII]) and the latter term to refer to the latter writings (roughly following WWII until the 1960s). However, with this strategy it must be noted that the differences are subtle and often difficult to keep clear when surveying the literature. Logical empiricists—for example, Carl Hempel (1905–1997)—tended to focus more on questions of methodology and were particularly concerned with establishing a theory of inductive logic. Inductive logic holds a central place in scientific investigation, yet it is by its nature an uncertain form of logic. Logical empiricists wanted to strengthen inductive logic in order to strengthen the force and certainty of scientific conclusions. The logical empiricists also furthered another study of the logical positivists: the distinction between observational and theoretical language. Theoretical language refers to terms that have no direct observational reference, such as atom, gravity, neutron, or electron. Given the strict empirical orientation of logical positivists/empiricists, 20th-century science’s continual use of such terms, referring to entities that are not or even cannot be directly observed, presented a problem. The distinction between logical positivism and logical empiricism is not a clear one. If there is a real conceptual difference, it is subtle and one of degrees rather than kind.

The first major challenge to logical positivism/empiricism came from the American philosopher W.V.O. Quine (1908–2000) in his 1951 paper “Two Dogmas of Empiricism,” published in the Philosophical Review. The two dogmas to which the title refers are the analytic–synthetic distinction, “a belief in some fundamental cleavage between truths which are analytic … and truths which are synthetic,” and reductionism, “the belief that each meaningful statement is equivalent to some logical construct on terms which refer to immediate experience” (Quine, 1951/2000, p. 115). These two “dogmas” relate directly to the two primary linguistic principles of logical positivism/empiricism: the analytic–synthetic distinction and what he called “reductionism” is an expression of the verifiability theory of meaning. Part of that theory is that we verify claims through direct and immediate experience. Regarding verifiability, Quine argued that it is impossible to verify (test) any single claim by itself. This is because the testing of any single claim is going to assume the truth of many other implicit claims, such as the claim that one’s testing apparatus is accurate. For example, in testing the claim that water boils at 100°C at sea level, one has to assume that a working thermometer is used and a working altimeter as well. In other words, if one gets a negative result in testing this claim, the claim itself might not be false, but one of the attendant assumptions might be false, like the assumption of the accuracy of the instruments. He argued for a more holistic approach to knowledge that asserts knowledge composed not of distinct, isolatable principles but an interconnected “web of belief.” If you change or cut one strand (belief), that will have an effect on other strands throughout the web.

Regarding the analytic–synthetic distinction, Quine asserted that the concept of analyticity is dependent on the concept of synonymy: that of the subject and predicate, one is defining of the other. Yet, synonymy itself cannot be defined without some reference to analyticity. Thus, we fall into a circular argument. Quine ultimately concludes, regarding this distinction, that “truth in general depends on both language and extralinguistic fact … for all its a priori reasonableness, a boundary between analytic and synthetic statements simply has not been drawn”—and with an obvious dig at the logical positivists—“That there is such a distinction to be drawn at all is an unempirical dogma of empiricists, a metaphysical article of faith” (Quine, 1951/2000, p. 123). The reason Quine’s blurring of this distinction is so devastating to logical positivism/empiricism is that those philosophers relied on the certainty and analyticity of logical and mathematical statements as a foundation for scientific method. Thus, again an approach that is less analytic and more holistic is affirmed by Quine.

Logical empiricism had to take Quine’s critique into account. The analytic–synthetic distinction was not rejected but became questionable. The verifiability theory of meaning had to be amended to make room for a more holistic interpretation (Godfrey-Smith, 2003). These changes, however, only postponed its inevitable decline. By the late 1970s, the school was effectively no more (Godfrey-Smith, 2003). Its decline was brought about by factors already noted plus others: (a) the breakdown of the logical positivist theory of language, (b) pressure from holist arguments, (c) a failure to develop a stronger theory of inductive logic, (d) a new recognition outside the school of the relevance of history and psychology to the philosopher of science, and (e) the aforementioned problem regarding the theoretical/observational language distinction that created a problem for the thesis of scientific realism (Godfrey-Smith, 2003).

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