Why do chemists think that the bond is real in spite of objections raised from the quantum mechanical studies of molecules? (Parr and Yang in Density-functional theory of atoms and molecules Clarendon Press, Oxford, 1994) Focusing on the cognitive aspect of investigative practices in chemistry, we reveal the meaning of the chemical bond for chemists and why they take it as real. Our argument is based on the historical studies of the bond and an understanding of the epistemological technique of transdiction as well. The latter is a way of reasoning developed since the days of Newton. The rationalist reconstruction of forces of nature is an illustration of transdiction as a tool of building theories. In the history of chemistry, a comparison between the notions of a directed valence proposed by van’t Hoff and an asymmetrical molecule by Le Bel shows transdiction has some methodological differences in application. In relation to transdiction we argue the meaning of the word ‘hypotheses non fingo,’ which serves as a key to understanding cognitive grounds for reality in experimental sciences. Our approach adopted in this study is characterized by functional realism we advocate in the context of organic chemistry. (Ochiai in Found Chem 26: 399–411, 2024) This approach reveals that the bond is a kind of function of the molecule that becomes actualized on occasions of solving problems in organic chemistry. Provided that reality is the epistemological concept, bonds are real for chemists in terms of function expected from chemical substances. It should be noted that the confusion between chemical ‘bond’ and quantum mechanical ‘bonding’ causes a lot of problems in understanding the concept of bond. The bond is a coded representation and not a visualization of quantum mechanical facts. Such is also the case with molecular structure, which is a functional map, as it were, to show what part of the molecule is susceptible to chemical transformations. (Ochiai in Hyle Int J Philos Chem 19: 139–160, 2013) It should be distinguished from the images of molecules obtained by using Scanning Tunneling Microscopy, for instance. This analysis shows that chemistry cannot be reduced to so-called more fundamental sciences like quantum mechanics, for the latter is not concerned with the functional aspects of chemical substances.
