The child is taught to accept “tree” and not “boojum” as the agreed sign for that (pointing to the object). We have no difficulty in understanding that the word “tree” is a matter of convention. What is much less obvious is that convention also governs the delineation of the thing to which the word is assigned. For the child has to be taught not only what words are to stand for what things, but also the way in which his culture has tacitly agreed to divide things from each other, to mark out the boundaries within our daily experience. Thus scientific convention decides whether an eel shall be
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