Racism and philosophy should both not need defining.
Presentism? Well, it has multiple definitions. I do not mean the philosophy version; although skepticism tells me I cannot prove this, I do not believe items in the present do not exist in the future and did not in the past.
Rather, I mean the literary-historical idea, which I am talking about for the sake of rejecting it, at least rejecting it on a case-by-case basis.
Namely, rejecting it on an ethical issue like racism.
It's interesting that, whether it's a certain subgroup of Southern whites and fellow travelers defending the Confederate flag, or certain sections of academia defending "the classical canon" from allegedly unwashed revisionists, that political conservatives, in trying to defend "traditional values" of some sort, resort to what is clearly itself relativism.
Because, if ethical and moral standards aren't constant, then traditional values mean nothing today. Just in their traditional past.
Even a fair chunk of liberals, though, wave the flag of presentism strongly. Perhaps they're afraid of being tarred as the latest incarnation of the new left.
That said, here's how I reject it. Let's look at the Wikipedia definition:
In literary and historical analysis, presentism is the anachronistic introduction of present-day ideas and perspectives into depictions or interpretations of the past. Some modern historians seek to avoid presentism in their work because they believe it creates a distorted understanding of their subject matter. …
Presentism is also a factor in the problematic question of history and moral judgments. Among historians, the orthodox view may be that reading modern notions of morality into the past is to commit the error of presentism. To avoid this, historians restrict themselves to describing what happened, and attempt to refrain from using language that passes judgment. For example, when writing history about slavery in an era when the practice was widely accepted, letting that fact influence judgment about a group or individual would be presentist, and thus should be avoided.
Critics respond that to avoid moral judgments is to practice moral relativism, a controversial idea.
Back to that issue of moral relativism, though.
It is true that "race" is largely a sociological construct. So, does that mean that racism, and a definition of such, is based purely on culture? Am I about to hoist myself on my own petard?
No and no.
First, while "race" is at bottom line a social construct, it's nonetheless one built on biological phenotypical markers. (Not genotypical; in different "black" ethnicities, different genes may be involved with details of melanin production; certainly, between three or four different groups of sub-Saharan Africans, plus the pre-Aryan "Dravidian" people of Indian, plus New Guineans and Aborigines, different genes are involved in hair differences, or to the degree they're similar, hair similarities.)
Second, a degree of xenophobia, and distinguishing between in-groups and out-groups, seems to be more in our genes, and those of our primate kin.
However, it is precisely culture that has shown us we can rise beyond basing in-group/out-group distinctions on a cultural construct called "race." Therefore, it is not "presentism" to wonder why Aristotle didn't do that. And certainly, closer to our times, it's not at all presentism to wonder about Hume, or his younger semi-contemporary Kant, who also had a racist bone or two.
It does mean that we should approach them carefully whenever they make race-related philosophical pronouncements. It means we should reject them as authorities on race outside of philosophy.
And, it means, since we're not engaging in presentism, we should ask how such otherwise intelligent people came to be racist in the first place.
(And we can and should remember that not all people who have racism issues are equally deep or virulent in their racism.)
Why that question?
Per another philosopher, Santayana: Those who don't learn from the past are condemned to repeat it.
As for fear of being "labeled" a presentist? Non-PC, non-SJW liberal friends, like Massimo, of mine should reject the use of this conservative cudgel. Since it's really a straw man, it's a cudgel without power unless you believe otherwise.