The term ontological fallacy has great currency in social philosophy, where it is used to denote the mistake of assuming that because there is a term for something, like a social institution, that the object it denotes really exists. A similar couple of fallacies are Whitehead’s “Fallacy of Misplaced Concreteness”, in his Science and the Modern World, which he engaged against a certain kind of scientific realism, and Marcuse’s “Reification Fallacy” in One Dimensional Man.
Whitehead’s version is well known:
“There is an error; but it is merely the accidental error of mistaking the abstract for the concrete. It is an example of what I will call the ‘Fallacy of Misplaced Concreteness.’ This fallacy is the occasion of great confusion in philosophy. It is not necessary for the intellect to fall into the trap, though in this example there has been a very general tendency to do so.” Science and the Modern World, 1926, p74f
A little later he says
“Thereby, modern philosophy has been ruined. It has oscillated in a complex manner between three extremes. There are the dualists, who accept matter and mind as on an equal basis, and the two varieties of monists, those who put mind inside matter, and those who put matter inside mind. But this juggling with abstractions can never overcome the inherent confusion introduced by the ascription of misplaced concreteness to the scientific scheme of the seventeenth century.” p82. Cf. also 85
However, I wondered who had come up with this fallacy. It is not listed in the Stanford Encyclopedia of Philosophy, nor in the Internet Encyclopedia of Philosophy, nor in my battered old copy of Dagobert Rune’s Dictionary of Philosophy, but you find it used all over the place, as early as the 1920s, as if everybody knew what it meant. Sometimes it appears that people are referring to the Fregean attack on the Ontological Argument, but this seems to be a casual use of the phrase.
A bit of hunting and I found what I think is the origin: an essay by F. C. S. Schiller in 1905, but he doesn’t use the phrase. Instead he takes F. H. Bradley through his disciple Alfred E. Taylor, author of Elements of Metaphysics (1903), to task for making two correlated errors:
In its essence, this would seem to be a form of the ‘ontological’ argument whereby a claim of our thought is turned into a revelation about reality. But in addition there is a twofold fallacy, viz. (1) an equivocation in the word ‘truth,’ which is used both of the internal self-consistency of thought and of its ‘correspondence with reality,’ and (2) the unworkable view of truth as the correspondence of thought with reality. [p357]
Schiller is a pragmatist, so he rejects both claims. It is a widespread error in philosophy to think that because we have, say, a clear and distinct idea of something, there must be a something to go along with it. It’s a kind of Word Magick. The world doesn’t much concern itself with matching our linguistic categories – in fact the causal arrow is, or at least should be, entirely the other direction.
The ontological fallacy is best known from Anselm’s rather abstruse argument that a conception of God must include all perfections, including existence, and so God exists. But reality doesn’t follow what we can conceive (and it is far from obvious that we can conceive of a fully perfect being, in my opinion, but that’s a different fallacy). Words should be attempts to make the world make sense. But as I discovered when I encountered the Universal Language Project, such as my namesake Bishop John Wilkins and Leibniz, among others, thought would capture the essence of reality, it is a longstanding fallacy, and a persistent one.
Schiller, F. C. S. 1905. Empiricism and the Absolute. Mind 14 (55):348-370.