I think a strong claim could be made that Descartes proffers an argument for epistemic justice. In short: if Descartes is correct, and the authority for knowledge claims comes from each individual making the claim for him or herself, then political and religious authorities are not to be taken, without further qualification, as sources for knowledge. Thus, the social forces pushing for a particular view of what is true and known are not to be taken as authoritative: rather, the individual is authoritative with regard to his or her own knowledge claims.
At a time when the church claimed special access to knowledge, and the Bible was accorded privileged place as a source of knowledge, Descartes held that this sort of testimonial knowledge from authority had no greater claim to truth, and indeed a much lesser claim to truth, than those claims that an individual derived from his or her own properly conducted investigations. Anything that the individual knew “clearly and distinctly,” then, had greater authority than any contrary claim coming from political or religious power.
It has been argued that Descartes’ thinking subject was inherently gendered, and gendered male (see for example Bordo’s “Cartesian Masculinization of Thought” 1986, Signs; Bordo’s Flight To Objectivity: Essays on Cartesianism and Culture, 1987, SUNY; Lloyd’s “Maleness, Metaphor and the Crisis of ‘Reason'” in Antony and Witt A Mind of One’s Own: Feminist Essays on Reason and Objectivity, Westview Press, 1993) but this is certainly not inescapably clear from the text. In fact, Descartes explicitly claims that the method is available to anyone.
Further, in an age when the church not only exacted terrible penalties for heterodoxy, using violence to assert its epistemic authority and priority, it also explicitly denied access to knowledge production and acquisition to women, insofar as the priesthood and thus the church hierarchy were closed to them. Descartes, operating outside this structure (though still, of necessity, appealing to it, as in his prefatory letter in the Meditations) offers a source of epistemic authority for anyone, male or female, willing to introspect.
Notably, among Descartes’ best known correspondences are those with Elizabeth of Bohemia. It’s clear in reading them that, unlike, say, Kant in his correspondence with Maria Von Herbert, Descartes never claims that Elizabeth is in any way deficient in reason due to her sex, nor considers her sex as part of her thinking substance. Where Kant refuses to engage philosophically with Von Herbert, offering her advice but not taking in her criticisms, Descartes is extremely mindful of the effectiveness of Elizabeth’s criticisms of his dualism. While he doesn’t give it up, and he engages in some hand-waving at the end, it doesn’t seem that he does so because she’s a woman.
If he was consistent with his own thought, of course, he would have to take her positions as subject only to a critique of reason, and not as coming from a biological determined source.
If we think of Descartes along these lines, as de-legitimizing an alleged epistemic authority that had, to use Miranda Fricker’s terms, “mere credibility” and “excess credibility,” and affording a kind of universal legitimacy to any person with regard to their own methodologically sound convictions, regardless of what anyone else says to them, we can see him striking an interesting blow for epistemic justice, one that is perhaps evinced in his (for the time) rather feminist tendency to take seriously the writings and philosophical thought of the women who corresponded with him.
In a sense, Descartes’ subject-centered epistemology sets the grounds for later developments that allow oppressed people to continue to assert the value of their subjective experience in the face of the negation of that experience by oppressive authority. They are sanctioned, by cartesianism, as the only true knowers of themselves, and needn’t pay attention to those who attempt to deny their claims from without.