Locke’s Mermaids
John Locke is rumoured to have confidently reported the existence of mermaids and seamen. It certainly appears so, according to this section from Book III, Chapter VI of his Essay Concerning Human Understanding:
There are fishes that have wings, and are not strangers to the airy region: and there are some birds that are inhabitants of the water, whose blood is cold as fishes, and their flesh so like in taste that the scrupulous are allowed them on fish-days. There are animals so near of kin both to birds and beasts that they are in the middle between both: amphibious animals link the terrestrial and aquatic together; seals live at land and sea, and porpoises have the warm blood and entrails of a hog; not to mention what is confidently reported of mermaids, or sea-men.1
The case is settled: Locke believes in mermaids—not to mention cold-blooded birds that both behave and taste like fish. Or it would be settled, if the above hadn’t been quoted somewhat selectively. Looking at the first and last sentences of the same paragraph:
Of finite spirits there are probably numberless species, in a continuous series or gradation. It is not impossible to conceive, nor repugnant to reason, that there may be many species of spirits, as much separated and diversified one from another by distinct properties whereof we have no ideas [...]
And yet of all those distinct species, for the reasons above said, we have no clear distinct ideas.
Locke’s primary concern in this section is pointing out that there are more 
things in the world than we already have ideas about. Book III in general is far more concerned with the relationship between language and ideas than with anything real in the world. Chapter VI, quoted from above, discusses essences; the section that mentions mermaids is part of an account of ‘nominal essences’, which by Locke’s definition are abstracted general ideas—bundles of properties sorted into kinds and referred to in general terms. The actual physical construction of a thing he refers to as it’s ‘real essence’, but that’s not what he’s discussing in the first quote above. Locke is merely discussing the nominal essence of a mermaid, as Chapter III, §19 shows:
And though there neither were nor had been in nature such a beast as an unicorn, or such a fish as a mermaid; yet, supposing those names to stand for complex abstract ideas that contained no inconsistency in them, the essence of a mermaid is as intelligible as that of a man; and the idea of an unicorn as certain, steady, and permanent as that of a horse. From what has been said, it is evident, that the doctrine of the immutability of essences proves them to be only abstract ideas; and is founded on the relation established between them and certain sounds as signs of them; and will always be true, as long as the same name can have the same signification.
In this earlier section, Locke is referring to nominal essences irrespective of what exists in the world. He concludes in the following section that the process of “men making abstract ideas, and settling them in their minds with names annexed to them, do thereby enable themselves to consider things”. Locke’s Essay explains how it is possible to conceive—as an abstract, general idea—of such a creature as a mermaid, but he’s certainly not claiming that there are such creatures.
CITATIONS:
1. John Locke, An Essay Concerning Human Understanding, Book III, Chapter VI, §12. [Online here.]
