I will certainly not be the first to find it interesting that some languages do not allow for a distinction between things and stuff. In Latin for example there is only res, a word that abounds with ambiguities, though some more easily soluble than others: almost every semester I get a paper or two purporting to analyze Descartes' theory of (sic!) race extensa and how it differs from race cogitans.
There is some ground for supposing that if Descartes had written in English or, say, German, he would have rendered the res that is extended as 'extended stuff', and the res that thinks as '(a) thinking thing'. That is, a res cogitans is understood to be a countable, individuable entity, while res extensa is an arbitrarily delimitable quantum of extension.
Now in the Germanic languages 'thing' and 'stuff' are partially overlapping but still quite distinct concepts. Here I am working from memory, for that is my way when I blog, but I believe I am correct in saying that 'thing' (German Ding, Icelandic þing, etc.) initially did much the same work as res in the phrase res publica: it picked out a composite, yet not arbitrarily delimited, political entity. In fact, the Icelandic alþingi is composed in exactly the same way as res publica: it is the thing that is constituted by all (or at least all who matter; on this point the Icelanders were rather more democratic than the Romans). What is important here is that a 'thing', whether in the political sense or in some more basic ontological sense, is a unit: a discrete, countable whole.
'Stuff' by contrast, if I am not mistaken, was originally (and still is in the German Stoff) a term that designated a quantity, in the primary instance a quantity of fabric. One buys some stuff, in order to transform it into a shirt or jacket, and this transformation can be understood as a sort of imposition of form on undifferentiated matter.
The analogy of stuff to matter is particularly interesting when we consider that for Aristotle there can be no un-in-formed matter waiting to be given form. Matter is thus only an abstraction, a limit-case that we can posit with our minds but that we cannot find anywhere in the world.
Of course we can find fabric, and this means that the analogy between Stoff and matter is only an analogy. Why the analogy works, though, may have something to do with the similarity, noted by P. F. Strawson and others, between mass terms and abstract terms. B. K. Matilal observes, for example, that "abstract terms are also 'much-terms', that is, the grammar of abstract terms, such as prettiness and courage, is similar to the grammar of mass terms" (Matilal, The Character of Logic in India, 25).
The correctness of this observation is borne out by a consideration of the uses of the partitive genitive in languages that rely on it: thus in French one can say Il a du courage, and one can ask Est-ce qu'il y a du lait?, as if courage and milk were both just two great masses of stuff from which one might draw one's share. Matilal explains, in fact, that in the Indian Nyāya school of logic a concept such as 'water' is treated exactly the same as the concept 'ability to swim'. Thus the proposition 'The water in this glass is cold', is in Nyāya transformationally equivalent to:
Water, which is characterized by being locatable, where such locatability is conditioned by a location-hood resident in the glass, is coldness-possessing.
The proposition 'John's ability to swim is poor', in turn, may be understood as follows:
The ability to swim is characterized by being locatable, where such locatability is conditioned by a location-hood resident in John, has the quality of being poor.
So courage, water, milk, fabric, ability: all of these are so much stuff, of which one can take what one needs or desires. All of these differ, in turn, from the countable beings that take of them. Whether this distinction is one that captures a real difference in the world or not is another question altogether, but it certainly feels helpful to have a lexical choice between 'stuff' and 'thing'.
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