Psychologism

Psychologism is a stance in the philosophy of logic that fundamentally reduces logical and mathematical truths to psychological facts. For instance, the psychologistic position holds that the laws of logic themselves carry no intrinsic meaning but are merely psychological laws describing how the mind thinks. Historically, those who advocated psychologism as an intellectual movement adopted this stance. This current of thought characterizes certain attitudes displayed by some thinkers in 19th-century Germany, where psychology courses were taught by philosophy professors and the science of psychology had not yet clearly distinguished itself from philosophy. Today, these debates are largely interpreted as a form of naturalistic philosophy.

The term is predominantly expressed in the German-language literature as Psychologismus. It is generally accepted that the first entry of this term into English occurred after 1870 through the English translation of the work by German philosopher Johann Eduard Erdmann. In his own work, Erdmann used the term Psychologismus, which was translated into English as Psychologism. Subsequently, the debate was shaped by interventions from Frege (1884) and Husserl (1891). Within this context, systematic debates between psychologism and logic are thought to have taken place between 1890 and 1914.^【1】

The earliest example of the phenomenon of psychologism, both etymologically and syntactically expressed, appears in the 1866 book Grundriss Der Geschichte der Philosophie^【2】, written by J. E. Erdmann and published in several editions thereafter as a summary of the history of philosophy. While discussing various schools, thought movements, and intellectual currents in the history of philosophy, the author criticizes the modern philosopher F. E. Beneke, who attempted to reduce logic to psychology, using the term Psychologismus.

Thus, the philosopher Beneke, who approached philosophy on the basis of psychology, is the originator of Psychologismus. Nevertheless, among the 19th-century representatives of the psychologistic movement, followers of Theodor Lipps, Christoph Sigwart, and Erdmann, including Erdmann himself, can be identified. In fact, although Erdmann warned of the dangers of psychologism, he himself endorsed its content, arguing that logical laws do not offer a fully independent idealism.^【3】

Within this atmosphere of psychological reductionism, the 1837 work Wissenschaftslehre^【4】 by Bernard Bolzano, which holds a central place in establishing the foundations of contemporary semantics and logic, would become of great importance to Husserl and Frege. Because of its use of the expression Sätze an Sich [Tur. Propositions in Themselves; Eng. Propositions in Themselves], this work argues for the existence of a realm of objective reality and, consequently, for the inherent normativity of logical and mathematical principles. It is regarded as initiating Husserl’s major philosophical project.^【5】 

The most prominent historical figure in the psychologism debates, Edmund Husserl, constructed a theory of mathematics in his 1891 book Philosophie Der Arithmetik by seeking the foundation of numbers in psychological processes. This work was sharply criticized by Frege, the author of Grundlagen Der Arithmetik^【6】, written seven years earlier. As commentators agree, Husserl clearly succumbed to psychologism in this early work.^【7】 However, in his later phase, Husserl played a key role in developing a discipline he named “phenomenology”—the science of appearances—drawing on the descriptive psychology terminology he inherited from his teacher Brentano.^【8】 Thus, in his third phase, Husserl fully identified phenomenology with the ontological relationship between lived experience and the world, having initially been sympathetic to psychologism but later decisively rejecting it in order to construct his own science.

Erdmann’s central argument in his book is a critique of F. E. Beneke’s position, which rejects the Kantian a priori philosophy that transformed Western philosophy in the 18th century and instead proposes a priori psychological developmental processes in place of a priori mental structures. According to Erdmann, Beneke’s aim was to ground the foundations of philosophy in empirical psychology and reduce the normative and objective structure of logic to an explanation of mental processes.

The mathematician Edmund Husserl, founder of transcendental phenomenology and author of the 1911 work Philosophie als Strenge Wissenschaft^【9】, also attacked psychologism in his Prolegomena zur reinen Logik^【10】 book, using the same reasoning. In this text, widely regarded as the most systematic critique of psychologism in history, Husserl makes a sharp distinction between the “psychological” and the “logical”:

Things in the logical space are divided into two: the first is “psychological,” the second is “purely logical.”... Everything that is “purely logical” is neither mental nor about acts, neither about the subject nor any other empirical subject or actual reality, but rather “ideal” things that are “in themselves” with their essential content [Wesensgehalt].^【11】

In subsequent passages, Husserl argues that the laws of logic are not psychological but ideal and normative, ultimately asserting that logic is a practical-normative discipline (Kunstlehre).^【12】 This normativity grants logic the authority of a theory of science (Wissenschaftslehre), and the knowledge generated by consciousness becomes its most fundamental unit.

Husserl’s passage here responds to a critique by Natorp in his 1888 book Einleitung in die Psychologie nach kritischer Methode [Introduction to Psychology from a Critical Method], which argues that psychology must be grounded transcendently. Natorp, as his central thesis, grounds consciousness in the subject, then psychology based on consciousness, and finally philosophy. Husserl, however, sought to ground philosophy in acts of consciousness, then in the domain of phenomena, and ultimately in a theory of knowledge through logic. For Husserl, logical laws were also ideal.

Natorp, however, maintained his original view in his 1910 work on the principles of logic, again grounding logical principles in consciousness. Thus, the aims and methods of the two thinkers differed. Indeed, Natorp, together with thinkers such as Wilhelm Wundt and Holzkamp who shared his views, helped establish an empirical discipline called “experimental/critical psychology.”^【13】

Another mathematician, Gottlob Frege, also argued in his 1884 book that arithmetic must be reduced to logic and developed arguments to support this project. Even in this less sharply delineated classification, Frege maintained that “mathematics” and “logic” can never be treated as psychological elements.

The position of many phenomenologists, beginning with Husserl, centers on Kant’s transcendental philosophy—the so-called Copernican revolution—as the first instance of psychologism. Kant, regarded as the founder of transcendental philosophy and a pivotal figure in the transformation of modern Western philosophy, argued in his 1781 work Kritik Der reinen Vernunft^【14】 that the principles of logic are fundamentally about the use of reason. According to him, the rules of thinking (understanding, Alm. Verstand, Eng. Understanding) are divided into two: rules of how thinking actually occurs, and rules of how thinking ought to occur. Logic is the science of universal rules of the use of reason.^【15】 Therefore, for Kant, logic is the name of the necessary relation between objects and consciousness, without regard to the nature of objects or direct contact with them.^【16】 Thus, Kant’s definitions of the logical are mental in nature.

Indeed, researchers who support this view, such as ^【17】, argue that Kant constructed a mind-based transcendental philosophy. However, other researchers reject the claim that Kant was a psychologistic thinker, arguing that psychologism involves equating logical laws with psychological laws, whereas for Kant, logical laws are not psychological or empirical but normative. Researchers examining Kant’s texts in context also agree that for Kant, logic is a transcendental structure while psychology is an empirical science.