Everyone agrees that ignorance of fact can excuse. If I take your suitcase thinking it was mine, and my belief that it was mine was faultless (perhaps the coach driver handed it to me, saying ‘this is yours’, and it looked exactly like mine), I seem excused of blame. But philosophers and ordinary people alike have been reluctant to think that moral ignorance can excuse. My ignorance that there was poison in the sugar bowl excuses me from blame, but few people are inclined to excuse on the basis of the claim that the person didn't know it was wrong to cause harms.

Obviously, it wouldn't be plausible for me to claim I didn't know it was wrong to poison you. But there are cases in which claims of moral ignorance seem more plausible. Debates about the morality of statues portraying people who made money in the slave trade often make these issues central. Those who want them torn down typically blame the people memorialized for their wrongdoing; conversely, those who defend the statues often think it's sufficient to deflect blame, on the grounds that the person was ‘a man of his time’, to make their case for the preservation of the memorials.

In this brief article, I want to draw attention to some undiscussed problems that arise when we blame those who acted horrendously at a time when acting in that way was widely accepted. If moral ignorance doesn't excuse, it must be different from factual ignorance in some important way. The common view is that it's different because it's much harder to maintain: it takes work to be morally ignorant, if it's possible to achieve it at all. The morally ignorant person must wilfully ignore the equal humanity of the members of other races or of women, say, to maintain their ignorance. Moral ignorance is different from factual ignorance because no one is innocently morally ignorant.

Suppose that's true. The problems I want to focus on arise when we ask of the wilfully ignorant: how could they act like that? How could they engage in such terrible actions? There are three possibilities: they acted so badly because they were worse people than us, or because they could get away with it, or due to factors outside their control. On each option, it's hard to see how they deserve blame, or at least deserve our blame.

Suppose the explanation for how they could act so awfully is that they were simply bad people; worse people than we are. Perhaps we've seen widespread moral improvement between their time and ours. Of course, there are plenty of terrible people around now – people we think would engage in awful conduct given the opportunity – but we can't explain slavery and genocidal colonization by reference to a small subset of awful people in the past. The worst offenders in the past might have been especially bad people – Christopher Columbus appears to have been seen by his contemporaries as barbarous – but most of those who engaged in the slave trade were not seen as especially bad. Their behaviour was normal and tolerated by almost everyone (the very existence of statues commemorating slave traders indicates how widely accepted their behaviour was). If we're to suppose that being worse people explains the slave trade, for example, we must suppose that typical people in the past were worse than most of us today.

The problem with this explanation is that it seems either to be mysterious or excusing. The obvious explanations of moral improvement between then and now all look pretty exculpating. If we've become better people because our relative affluence allowed us the luxury of treating others as persons and not merely threats, or because improvements in communication allowed us to understand one another better, or due to an institutional environment in which trust is warranted, we've become better people due to facts for which we're not responsible. This looks exculpatory, because we've explained the badness of past people by reference to facts that neither they nor we are responsible for (facts the significance of which they might not even have been able to see). On the other hand, if there's simply no explanation of why we're better people than they are, if it's just a brute fact, that's very mysterious, and our theories shouldn't appeal to the existence of mysteries.

Suppose we're not morally better than them. Why did they act worse than we do? If we're no better than they are, then it seems to follow that we would have acted just as badly as they did in their circumstances. The only difference between them and us is that they had the opportunity to act like that. If that's right, though, then our standing to condemn them is considerably weakened. In general, one agent can't justifiably blame another for acting badly if the first agent would have acted just as badly had they the chance to do so. So even if they're blameworthy, we're in no position to blame them and we should not do so.

All in all, it's hard to make sense of the situation where ‘ordinary’ past wrongdoers are deserving of our blame. Either they're genuinely and non-culpably morally ignorant, which seems to be excusing for the same reasons as factual ignorance is excusing, or they're excused on the grounds that they were bad people for reasons they couldn't help, or we lack standing to blame them.

Of course, slave owners, and those who supported them, were engaged in morally horrendous activities. Of course, we should condemn what they did. That seems adequate grounds to tear down the statues. We don't need to blame them as well.