Valéria asked me whether I thought the British empire was a good thing.
There are all sorts of interesting things one can say about the good and bad things the British did. But ultimately to answer a question like this, which is a very reasonable one to ask, we need to compare what actually happened with what would have happened supposing that the Empire had not come into being. Which of these two courses of history would have been better? This is just the same sort of question political scientists are always asking and answering. For example, after performing a logit regression (a piece of statistics where the dependent variable is binary, for example, did a particular person turn out to vote or not?) we may say: if this person had been a woman instead of a man, but everything else had been the same - if she had had the same social class, education, religiosity and so on - she would have been ten per cent more likely to vote. Maybe in some cases the regression will be misspecified or based on inaccurate data or whatever. But in principle this can be valid.
If our historical knowledge - the application of our social science knowledge - is good enough, we can answer counterfactuals about history. For example, we can guess that Hong Kong would have been worse off if it had been colonized by the Portuguese and not by the British. We can make this guess without a deep understanding of history, because Macau, a small island rather like Hong Kong, actually was colonized by the Portuguese, and has not to date been nearly as economically successful as Hong Kong. Again, our guess is not infallible. Perhaps there is something we don't know that is specific to Hong Kong. But it is a reasonable guess.
Can we make the same kind of assessment of the Empire as a whole?
To do that we need to imagine an alternative history in which the British Empire does not come into being. Several problems at once present themselves. First, we need a coherent story. Why do the British fail to colonize large parts of the world in the 19th century? Well, they could have been unable to, if they had not had an industrial revolution. But now we are answering a very different question: what would have happened if the British had not had an industrial revolution? The consequences for human history would have been much broader than the question of empire. Or perhaps, the British simply chose not to pursue their empire. But then we run into a worry about psychological plausibility. For humans not to pursue power and empire when they can runs counter to the general tendency of most of history. If we assume this is what happened, are we assuming a different kind of humanity? Gentler and kinder, or simply more prone to take its own religious ideology (the Sermon on the Mount, say) seriously? Again, that's a different question from Valéria's question about the Empire.
It is not that we cannot coherently imagine histories without a British Empire. We can imagine many. The problem is that none of them will cut the Empire neatly out of the world without cutting a great deal more besides. Another problem is that there are very many of these histories: we do not have a useful criterion for choosing which one to use as a point of moral comparison. And many of these histories will be incomplete (no use for moral evaluation) without answering a lot of questions which, again, are irrelevant to what Valéria wants to know. For example, we could imagine that the French defeat the British in the eighteenth century struggle for power. Then we will need to know whether there is a counterfactual French empire, how far it will extend, and what it will be like. Our answers to these questions will determine how we evaluate the counterfactual history. Finally, when we construct our alternative history, we need to know what the British themselves were doing (instead of having an empire). After all, they were important actors in world history. To keep our counterfactual relevant, we will need them to be doing almost exactly what they actually did, except without having an empire. But this is rather like imagining a vertebrate except without its backbone. The empire was central to what the British were doing at the time. Again, we can always think of other things they might have done, but the problem is that there are too many of these alternative courses of action, all of them with widely differing impacts on history and human happiness.
Small-scale "what ifs" are answerable and important. (That's why I believe in social science and not just in, say, narrative history.) It's easy to believe that large scale "what ifs", which look just the same grammatically, must be equally meaningful. But these considerations lead me to believe they are not. At some point of scale, we lose the ability to construct the right kind of counterfactual.