The Ontological Status of Theoretical Entities

The Ontological Status of Theoretical Entities

That anyone today should seriously contend that the entities referred to by scientific theories are only convenient fictions, or that talk about such entities is translatable without remainder into talk about sense con- tents or everyday physical objects, or that such talk should be regarded as belonging to a mere calculating device and, thus, without cognitive con tent-such contentions strike me as so incongruous with the scientific and rational attitude and practice that I feel this paper should turn out to be a demolition of straw men. But the instrumentalist views of out- standing physicists such as Bohr and Heisenberg are too well known to be cited, and in a recent book of great competence, Professor Ernest Nagel concludes that “the opposition between [the realist and the in- slrumentalist] views [of theories] is a conflict over preferred modes of sp cch” and “the question as to which of them is the ‘correct position’ ha s only terminological interest.” 1 The phoenix, it seems, will not be laid to rest.

The literature on the subject is, of course, voluminous, and a compre- lt nsive treatment of the problem is far beyond the scope of one essay. I sl1all limit myself to a small number of constructive arguments (for a r lically realistic interpretation of theories) and to a critical examination of s me of the more crucial assumptions (sometimes tacit, sometimes · pli it) that seem to have generated most of the problems in this area.2

‘ fo: . Nngcl, TJ1c Structure of Science (New York: Harcourt, Brace, and World, l ‘) il), h . 6.

1 l•’or th e ge nes is and part of the content of some of the ideas expressed herein, I 11n ind ·bled to a number of sources; some of the more influential are H. Feig!, ” 11: IN! ·11t inl llypotheses,” PI1ilosophy of Science, 17 : 35-62 ( 1950); P . K. Feyerabend , ” 11 Alt · 111pt nt n Rcnlistic Interpretation of Experience,” Proceedings of the Aristo- 1 / 1111 Soi ty, 58 :144- 170 (1958); N . R . Hanson, Patterns of Discovery (Cam- 111 ii 1: ,n111hridgc University Press, 1958); E. Nagel, Joe . cit.; Karl Popper, The I 11 c• of S ·i 11tilic Dis ovcry (London : Hutchinson, 19 59); M. Scriven, “Definitions, f1,ph11111 t i11 11 s 1 :ind Th ·ori ·s,” in Miuneso ta Studies in tlie Philosophy of Science,


Grover MaxweII

The Problem Although this essay is not comprehensive, it aspires to be fairly self-

contained. Let me, therefore, give a pseudohistorical introduction to the problem with a piece of science fiction (or fictional science).

In the days before the advent of microscopes, there lived a Pas teur- like scien tist whom, following the usual custom, I shall call Jon es. Re- fl ecting on the fact that certain diseases seemed to be transmitted from one person to another by means of bodily contact or b y contact with articles handled previously by an afHicted person, Jones began to specu- late about the mechanism of the transmission. As a “heuristic crutch,” he recalled that there is an obvious observable mechanism for transmis- sion of certain afHictions (such as body lice), and he postulated that all, or most, infectious diseases were spread in a similar manner but that in most cases the corresponding “bugs” were too small to be seen and, pos- sibly, that some of them lived inside the bodies of their hosts . Jones pro- ceeded to develop his theory and to examine its testable consequences . Some of these seemed to be of great importance for preventing the spread of disease.

After years of struggle with incredulous recalcitrance, Jones managed to get some of his preventative measures adopted. Contact with or prox- imity to diseased persons was avoided when possible, and articles which they handled were “disinfected” (a word coined by Jones) either by means of high temperatures or by treating them with certain toxic prepa- rations which Jones termed “disinfectants.” The results were spectacular: within ten years the death rate had declined 40 per cent. Jones and his theory received their well-deserved recognition.

However, the “crobes” (the theoretical term coined by Jones to refer to the disease-producing organisms) aroused considerable anxiety among many of the philosophers and philosophically inclined scientists of the day. The expression of this anxiety usually began something like this: “In order to account for the facts, Jones must assume that his crobes are too small to be seen. Thus the very postulates of his theory preclude

V~I. IT , TT . Feig!, M . Scri~en ~ and G . ~axw~l.1 ‘. eds . (Minneapolis : University of !”111111. s tn Pre~s. I ?58); W1lfn~ Sellars, Empmc1sm and the Philosophy of Mind,” 111 M11111 so tn t11d1 cs m the Philosophy of Science, Vol. I, H. Feig! and M . Scriven , ~ I s. ( ,M i::n. npo lis: niversity of M in.nesota Press, .19 56), and ” The Language of 111 ·0 11 ~. 111 urr ·11t I ssues m the P/11losophy of Science, H . Feig! and G . Maxwell,

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