In forty-odd years of looking into works of philosophy now and then, I don't remember to have encountered any remarks on red hair. Then yesterday, in Note B, "Relation and Quality" in the appendix to F.H. Bradley's Appearance and Reality, I encountered
Two men with red hair for example, it may be urged, are either not related at all by their sameness, or when related by it are not altered, and the relation therefore is quite external. Now if I suggest that possibly all the red-haired men in a place might be ordered to be collected and destroyed, I shall be answered, I presume, that their red hair does not affect them directly, and though I think this answer unsatisfactory, I will pass on.
Beginning on the facing page, the case of the red-haired men, and their relation or not, gets another three paragraphs, amounting to more than a page.
This afternoon I took up The Structure of Appearance by Nelson Goodman, a work published a half century after Bradley's and in a wholly different tradition. On page 4, Goodman discusses the notion of extensional identity:
We do not require that the definiendum and the definiens agree with respect to all cases that 'might have been' as well as to all cases that actually are. For example, if all and only those residents of Wilmington in 1947 that weigh between 175 and 180 pounds have red hair, then "red-haired 1947 resident of Wilmington" and "1947 resident of Wilmington weighing between 175 and 180 pounds" may be joined in a a constructional definition (assuming, of course, that all terms in the expression taken as definiens have been previously introduced into the system).
I can see that I will have to be on the watch for red-haired men when I read philosophy.