According to Aristotle’s Politics, humans are both rational and social animals. I wonder from what time onwards rationality and sociality were taken to be related. Of course you might think that they are not related in any interesting way. But it is clear that some philosophers took them to be related. One way of putting the problem is to combine these properties in a priority question: What is prior to what? Is rationality prior to sociality or is it the other way round. Is it reason that makes us social or is it sociality that makes us rational?
According to many medieval authors, human rationality is taken to be independent from human sociality. That’s why Aquinas, for instance, famously says that, if humans weren’t also social, they wouldn’t need langage on top of concepts. By contrast, people like Thomasius and Kant claim that rationality might depend on sociality. If we weren’t social, we couldn’t think (properly). So, reason is taken to be, to some extent, social (and therefore linguistic). Now this shift of emphasis seems to become more pronounced in the 18th century, but my hunch is that it might originate at least in Grotius and Pufendorf, who make socialitas the (anthropological) basis of natural law.
But I wonder whether this might not have some roots in earlier authors. Humanist and Renaissance philosophers might come to mind, but not to mine… So if you have any ideas regarding sources about the social nature of reason, please send them my way.