This epistemological position is known as physicalism and is upheld by behaviorism, which considers only observable behavior as a legitimate object of study. John B. Watson, widely recognized as the founder of behaviorism, articulated this view in his 1913 article, Psychology as the Behaviorist Views It, often referred to as the “behaviorist manifesto.”

Psychology as the behaviorist views it is a purely objective experimental branch of natural science. Its theoretical goal is the prediction and control of behavior. Introspection forms no essential part of its methods, nor is the scientific value of its data dependent upon the readiness with which they lend themselves to interpretation in terms of consciousness.

(Watson, 1913, Psychological Review, p. 158)

In psychology, the idealist perspective was most prominently reflected in the work of Wilhelm Wundt, often regarded as the father of experimental psychology. Through systematic introspection, Wundt and his students sought to break down conscious experience into basic sensory and affective elements, aiming to study the structure of the mind by analyzing conscious experience. Their introspectionist method emphasized that reality, as experienced, is inherently tied to subjective phenomena. Wundt argued that the content of consciousness is the only reality directly accessible to the observer:

Psychology is the science of immediate experience. The task of psychology is to investigate the connection of conscious processes according to the laws of experience.

Wilhelm Wundt, 1896

However, the Kantian idea that we construct our own world has continued to have an impact right up to the present day. A modern incarnation of such is known as radical constructivism (a radical form of constructivism), with representatives like Paul Watzlawick arguing that reality is nothing more than a subjective construction.

Hallucinations provide a compelling example of the distinction between external and internal perspectives. For individuals experiencing them, hallucinations feel undeniably real and encounterable—for example, many patients with schizophrenia hear voices that seem physically present. However, from an external perspective, these hallucinations cannot be perceived by others—they cannot be heard, experienced, or interacted with. Thus, hallucinations are real for the subject. From an external point of view, however, they exist only within the perceptual and cognitive framework of the individual experiencing them.

One well-established psychological theory that seeks to explain hallucinations is the Source Monitoring Framework. According to this framework, hallucinations arise from a failure to recognize certain cognitions or percepts as self- or internally generated. For example, verbal auditory hallucinations are hypothesized to result from an inability to identify inner speech as self-produced, while visual hallucinations may stem from the externalization of mental imagery. Consequently, individuals prone to hallucinations are considered impaired in their ability to distinguish between imagination and perception—referred to as reality discrimination—leading to a bias toward misattributing internally generated cognitive events to an external source.

We will learn more about hallucinations and what they reveal about how perception functions in Chapter 4. Some theorists even suggest that all of perception may be understood as a form of controlled hallucination—a perspective we will return to and examine in greater depth later in the course.