"Different Men . . . have different Essences of Gold, which must therefore be of their own, and not of Nature's making"(Essay   III.vi.31)
What did Locke mean by this?
Does it imply that the classifications that we make of things in the world are arbitrary?

In his Essay Concerning Human Understanding(1), Book III, Chapters iii thru v, John Locke introduces his distinction between "nominal essences"and "real essences".   He argues that the concept of gold, an example of what is supposedly a natural kind, actually denotes a nominal essence, and not a real essence as most people assume.

Locke's discussion of essences takes place against the historical background of Scholastic metaphysics.   One of Locke's main aims in his Essay was to provide a sound philosophical foundation for the "Corpuscular Hypothesis"of Boyle.   In Locke's judgement, the then new mechanistic / corpuscularian (atomic) physics was the best science available.   This was in specific contrast with the then traditional Scholastic notions of the teleology of Aristotelean "formal"and "final"causes.   The Scholastics based their notion of a "sort"or a "kind"of thing on a vaguely defined notion of "essence".   The Scholastics viewed the "essence"that defined a "sort"or a "kind"as something real -- something actually a part of a thing that made it a member of the kind or sort of thing it was.   Hence they thought of "kinds"as deriving from a natural basis -- "natural kinds" -- the kind being established by nature prior to and independent of how we think of things; the kind being an integral part of things prior to and independent of how or whether we think of them.   The exploration of the nature and account of this notion of "essence"was a widely debated topic in philosophy in the years before Locke wrote.   Locke's attitude was that this debate, as the Scholastics understood it, was pursuing a misconception.   Locke argued that contrary to the Scholastic notion that "essence"was a part of nature, in fact the "essence"that defined a kind or sort was simply an abstract general idea.   It specifically was not, Locke argued, something inherent in the things being sorted.

To Locke, "essences"are simply the general terms denoting abstract ideas created by the mind.   The abstract ideas are the meaning of those general terms.   Hence the essence of "gold"is simply the abstract general idea of gold created by the mind.   We presume the essence of gold is immutable because of our ability to retain the content of the abstract idea of gold independently of any particular sample of the substance [Essay III.iii.19-20].   Since one's own ideas are inaccessible to others, we employ the symbols of language as the sensible means of transmitting our ideas to others.   According to Locke, the absence of any universal language proves that the connection between linguistic symbols and the idea signified is not natural but purely conventional.   It is an association established by "voluntary imposition." [Essay III.ii.1-2]   The autonomy which individuals enjoy in forming their symbol-idea associations frequently entails difficulties for a sender of the symbol in ensuring that the appropriate idea has actually been induced in a hearer. [Essay III.ii.6-8]

Because abstract general ideas are formed by the mind of each individual, and the ideas within any particular mind are inaccessible to other minds, abstract ideas can therefore easily differ from mind to mind.   Hence, the essay's title quote that different persons have different abstract general ideas of "gold"- which are necessarily their own and inaccessible to the minds of others, and are "not of Nature's making".   The truth of Locke's understanding of abstract general terms is demonstrated by the frequency of disputes over the applicability of some general term to specific existents(2).  

Variations of word meaning from one mind to another are easily tolerable, of course, as long as one person's meaning is close enough to the other person's that the message doesn't get too badly garbled.   But that is what learning a language (or new words in a language)   is all about, after all -- ensuring that one's abstract general idea is close enough to what is expected, that messages do not often get garbled.

Locke denied that our creation of abstract general terms is ever governed by reference to the Scholastic notion of natural kinds.   Our abstract general terms are created entirely for our convenience in communicating ideas from one mind to another.   In the case of substances like gold, Locke argued that the belief that our word "gold"must be given meaning by reference to a natural kinds or substantial form of "gold"is based upon a pair of false suppositions: (i) that there are such natural kinds,   and (ii) that we have knowledge of them [Essay III.x.20-21].   Thus, on Locke's view, the classification of particular things into sorts or kinds, denominated by general terms, has no direct foundation in nature.   It is, instead, the end result of our own complex process of abstraction. [Essay III.iii.12-14]

In some, this complex idea [of gold] contains a greater, and in others a smaller number of qualities; and so is apparently such as the mind makes it. The yellow shining colour makes gold to children; others add weight, malleableness, and fusibility; and others yet other qualities, which they find joined with that yellow colour, as constantly as its weight and fusibility. For in all these and the like qualities, one has as good a right to be put into the complex idea of that substance wherein they are all joined as another. And therefore different men, leaving out or putting in several simple ideas which others do not, according to their various examination, skill, or observation of that subject, have different essences of gold, which must therefore be of their own and not of nature's making.     [Essay   III.vi.31].

What exactly is it, according to Locke, that a general abstract classification term like "gold"signifies?   It cannot refer to particular instances of gold, because we use the word to refer to gold generally.   It also cannot refer to a plurality of pieces of gold, because we distinguish between a single sample of gold and many samples of gold -- same word for one and many.   Locke argues that what general terms like "gold"signify are "sorts".

In claiming that it is our abstract general ideas that define sorts or kinds, rather than the other way around as the Scholastics would have it, Locke claimed that it is the human mind that creates the general idea, so it is the human mind that creates kinds or sorts.   In other words, it is not Nature that makes the natural kind "gold", it is each individual's abstract general idea that picks out whatever we choose to call "gold".   The world is not populated by natural kinds that we discover and name.   There is only the continuum of varying particulars.   We create the kinds and sorts to suit our convenience.   Hence Locke applies the name "nominal"to this kind of "essence".

Locke then distinguishes between real essences as contrasted with nominal essences.   By the "nominal essence" of gold Locke means just that collection of observable properties (yellow, malleable, etc.) that comprises our abstract general idea of gold.   But in addition to these observable properties, however, Locke proposes that there is also the unobservable microstructure that gives rise to those observable properties.   This microstructure is the consequence of the Corpuscular Hypothesis of Boyle and the then new "mechanistic"physics.   This internal (and to him invisible) constitution of objects is what Locke refers to as their "real essence."   Unlike the nominal essence of gold, the real essence of gold has a basis in reality.

It is on the basis of the real essences that Locke argues that the classifications that we make of things in the world are not arbitrary.   According to Locke, we base our abstract general ideas (our identification of nominal essences) on the observable properties we see.   These observable properties are really in the world and are caused by the microstructure of things (the real essences).   The similarities and differences between particulars really do exist.   But we choose which similarities and differences matter, which similarities and differences are relevant to our purposes.

Locke argues that our identification of "sorts"or what we mistakenly call "natural kinds"is based on nominal essences and not on real essences.   Firstly, because we have had our sortal words (and have already formed the associated abstract general ideas) long before we knew anything about the internal corpuscular structure of things (the real essences).   And secondly, because we do not (or at least did not in Locke's time) know the real essences of things -- we do not (or at least did not then) know enough about the corpuscular microstructure of things and how that structure generates the observable properties upon which we base our classifications.   Finally, Locke argued that the corpuscular microstructure of things gives rise to all of the observable qualities -- both those we deem relevant (and use in our classifications) and those we ignore as irrelevant (and perhaps of which we are not conscious).  

It is on the basis of our cognitive purposes that we identify qualities that we judge relevant, and thereby pick out the nominal essences, sorts, and kinds of things in the world.   The selection of which similarities and differences in observable qualities are relevant to our purposes comes before the identification of which part of the microstructure causes those observable features.   And that means that the real essence of gold (whatever it may be) is dependent on the nominal essence of gold.   Even if, as Kripke argues, our term for the abstract general idea (like "gold") is intended to pick out only that which has a particular real essence(3), it remains the case that the real essence is determined by the nominal essence, and not the other way round.

In other words, a "natural kind"like gold is dependent on our conventional ideas of what "gold"is, and not on any underlying "natural"nature of the stuff we choose to classify as gold.   It is not the case that "gold"always has and always will denote the material that is the element with atomic number 79.   It is rather the case that those lumps of stuff that we have already classified as "gold"turn out to have atomic number 79.   So the classifications that we make of things in the world are not arbitrary.

So, according to Locke's theory of essences and abstract general ideas, each individual forms their own abstract general idea -- an idea of a nominal essence of some stuff -- for a particular general term like "gold".   It is a consequence of the need for successful communication that drives a close similarity in the content of each individual's idea of the nominal essence of the stuff being discussed.   But that closeness need not be identity.   It need only be close enough to avoid most confusions.   For all the individuality of each person's idea of the nominal essence of gold, there is an underlying real essence of the material so picked out that causes all of the observed properties.   Therefore, the similarity observed by each individual, among multiple samples of gold, is not arbitrary.  

References

(1)   Locke, John (Author) & Yolton, John W. (Editor);   Essay Concerning Human Understanding, Everyman's Library, Dent Publishing, London England, 1961. ISBN: 0-460-00332-1.   All quotations from the Essay will be cited as [Essay book.chapter.section] of this edition.

(2)   Murphy, Gregory L.;   The Big Book of Concepts, A Bradford Book, The MIT Press, Cambridge, Massachusetts, 2004.   ISBN 0-262-63299-3. Chapter 2.

(3)   Kripke, Saul A.;   Naming and Necessity. Harvard University Press. Cambridge, Massachusetts. 1980. ISBN 0-674-59845-8.

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