Rudolf Carnap (1891-1970) was a philosopher and logician who was born in Ronsdorf, Germany. He emigrated to the United States in 1936 and became a U.S. citizen in 1941. He taught at the German University, Prague from 1931-5, at the University of Chicago from 1936-52, and at the University of California, Los Angeles from 1954-62. He died in Santa Monica, California in 1970. His writings included Der logische Aufbau der Welt (1928, The Logical Structure of the World & Pseudoproblems in Philosophy, 1964), Logische Syntax der Sprache (1934, The Logical Syntax of Language, 1937), The Logical Foundations of Probability (1950), and Meaning and Necessity: A Study in Semantics and Modal Logic (1956).

The Logical Structure of the World explains how a logical system of concepts can be based on, and can be reducible to, whatever is immediately given by direct experience. This approach to structural analysis is a kind of logical empiricism, affirming that all scientific statements are reducible to structural statements about basic elements of experience. Structural statements are logical propositions about the formal properties of objects or relations. The formal properties of objects or relations, and not their material properties, determine their applicability to a logical construction of the world.

Carnap uses the term "structure" (Aufbau) to refer to all the formal properties of an object or relation. The formal properties of an object or relation are those properties that define its logical structure but not its material content. Scientific statements may describe the structural properties of objects or relations, and they may take the form of property-descriptions or relation-descriptions. A property-description may be concerned with the formal properties of an object (or concept, statement, or relation). A relation-description may be concerned with the relations that occur between objects or the relations that occur between other relations. A relation-description may also take the form of a structure-description, which may indicate the relations that are sufficient to determine the structure of an object or the relations that are sufficient to determine the structure of other relations.

Carnap defines a constructional system of objects as a system in which the objects of each level are constructed from objects of more elementary levels of construction. The most elementary level of a constructional system is the level of basic objects, which includes basic elements and basic relations. The basic elements of a constructional system may be defined as "elementary experiences," which are not constructed and are immediately given as formal objects.

The basic objects of a constructional system may be divided into those that are autopsychological (eigenpsychisch) and those that are heteropsychological (fremdpsychisch). Autopsychological objects are objects of a person’s own experience, but heteropsychological objects are objects of experience for other persons. Autopsychological and heteropsychological objects may be objects of subjective experience, but if their formal properties are investigated, then they may also become objects of intersubjective experience. Thus, scientific investigation of the formal properties of objects and relations may transform subjective experience into intersubjective experience, and it may transform intersubjective experience into objective knowledge.

Intersubjective correspondence of autopsychological and heteropsychological objects may occur if the objects of a person’s own experience correspond to objects of experience for other persons. Thus, the intersubjective object domain is a unified domain of the autopsychological and the heteropsychological. This unity of the object domain is an important aspect of the logical unity of a constructional system of reality.

Because the validity of scientific statements about the structure of objects can be empirically confirmed and verified, there can be intersubjective agreement about the properties and relations of the objects of a constructional system. Thus, the construction of a logical system of physical or psychological objects can provide knowledge of the intersubjective reality of the objective world.

Two problems that may need to be solved in order to better understand the relations of objects in a constructional system are described by Carnap as the "correlation problem" and the “essence problem.” The "correlation problem" is the problem of determining the objects that are involved in a given relation, while the "essence problem" is the problem of determining the nature of the relation between related objects. The "correlation problem" may be a scientific problem, but the "essence problem" may be a metaphysical problem.¹

Another problem that we may need to consider is described by Carnap as the "psychophysical problem." The psychophysical problem is that of what constitutes the nature of the "psychophysical relation." The psychophysical relation is the relation between a psychological process and a corresponding physical process (i.e. between a sensory perception and a corresponding neurophysiologic process). The psychophysical problem, according to Carnap, may be an "essence problem."² The essence of the psychophysical relation may be a problem for metaphysical inquiry, and it may not be a problem that can be answered scientifically.

The types of objects that may be found in a constructional system include physical, psychological, and cultural objects. Physical and psychological objects may be reducible to each other, and they may occur at multiple constructional levels. Cultural objects may be constructed from, and may be reducible to, psychological objects. The mutual reducibility of physical and psychological objects is defined by the fact that statements about physical objects may be transformed into statements about psychological objects, and by the fact that statements about psychological objects may be transformed into statements about physical objects. Physical and psychological objects may be connected by the "psychophysical relation" (whereby a psychological event is produced by a corresponding physical event) or by the "expression relation" (whereby a physical event expresses a psychological event). Other types of objects that may be found in a constructional system include logical, mathematical, and spatial-configurational objects.

Objects may be autonomous if they each have their own sphere. The "sphere" of an object may be defined as the class of objects that are permissible arguments for the same argument-position as that object in any propositional function.³ A propositional function is an incomplete statement that is lacking one or more arguments from its argument-positions. If the names of objects are inserted into the argument-positions of a propositional function, then that propositional function may become a complete statement. A propositional function that contains only one argument-position may define a property. A propositional function that contains more than one argument-position may define a relation. Objects may be "isogenous" if they are permissible arguments for the same argument-position in a propositional function.⁴ "Isogenous" (sphärenverwandt) objects share the same object-sphere, but "allogeneous" (sphärenfremd) objects do not. Thus, any differences in the spheres to which allogeneous objects belong must be recognized if logical error is to be avoided.

The basic elements of a constructional system cannot be analyzed into their proper constituents. "Elementary experiences" are unanalyzable, because they are not constructed and are immediately given to consciousness as the basic elements of a constructional system of reality. Thus, the basic elements of a constructional system of reality cannot be given property-descriptions and can only be given relation-descriptions.⁵ The basic elements of a constructional system of reality may not be accessible to proper analysis, and they may only be accessible to a quasi-analysis that reveals their quasi-constituents. Objects at higher levels of construction may be analyzed, but proper analysis of these objects may only proceed until their basic elements have been identified, and further analysis may then be only a quasi-analysis.

Carnap explains that application of the rules of a constructional system may be useful for resolving many philosophical problems. For example, the rules of a constructional system may resolve the problem of what constitutes the essence of an object, because they may clarify the difference between constructional essence and metaphysical essence. While the constructional essence of an object may be defined by how the object is constructed from basic elements or relations, the metaphysical essence of an object may be defined by the inherent being of the object or by the being of the object as a thing-in-itself.⁶

The rules of a constructional system may also clarify the problem of whether the mind and body are two different substances. According to construction theory, physical and psychological objects are reducible to the same basic elements and relations, and thus they do not arise from different domains. Construction theory may also clarify the problem of what defines the nature of the self, because the self may be defined as a unified expression of elementary experiences.⁷

According to Carnap, construction theory may also clarify the problem of what defines the nature of intentionality, because the intention relation may not be a unique relation that refers to something outside of itself, but may be a subclass of relations between a given experience and an experiential structure in which that experience is included.⁸

Carnap emphasizes that construction theory is based on the empirical reality of physical and psychological objects, and not on their metaphysical reality. Construction theory does not assert that physical and psychological objects have an objective reality that is independent of their empirical reality as objects of consciousness. If physical and psychological objects had this kind of non-empirical reality, then their reality would not be constructed by cognition and could not be empirically verified. However, construction theory agrees with epistemological realism in affirming that real experiences are objectively different from unreal experiences, and in affirming that experiences can become objects of knowledge only insofar as they are real.⁹

Construction theory agrees with transcendental idealism in affirming that empirical objects may be constructed as concepts, and in affirming that only as constructed concepts can physical or psychological objects be integrated into a system of knowledge.¹⁰ Construction theory also agrees with phenomenalism in affirming that empirical objects are logical constructions that are based on elementary experiences, and in affirming that objective knowledge is limited to whatever can be constructed from elementary experiences of the world. Thus, Carnap says that construction theory does not contradict the epistemological claims of realism, transcendental idealism, or phenomenalism.¹¹ However, he emphasizes that it does not support any of the conflicting metaphysical claims of these different schools of thought.

¹Rudolf Carnap, The Logical Structure of the World & Pseudoprolems in Philosophy, translated by Rolf A. George (Berkeley: University of California Press, 1969), pp. 35-6.
²Ibid., p. 37.
³Ibid., p. 52.
⁴Ibid., pp. 51-2.
⁵Ibid., p. 111.
⁶Ibid., p. 256.
⁷Ibid., p. 260.
⁸Ibid., p. 299.
⁹Ibid., p. 284.
¹⁰Ibid., p. 285.
¹¹Ibid., p.284.

Carnap, Rudolf. The Logical Structure of the World & Pseudoproblems in Philosophy. Translated by Rolf A. George. Berkeley: University of California Press, 1967.