But if the issue is so simple, why are others making mistakes about it? Here are a few possible explanations:
(A) We are used to scientific properties, e.g. force and momentum, where action at a distance - let alone a distant time! - is considered "spooky". One might think of harm analogously as a kind of metaphysical "stuff" that gets "transmitted" from the cause to the recipient of the harm. I then seem committed to weird time-travelling stuff. (Hint: ethical relations don't need to move! There's no "stuff" getting "transmitted", except in a very metaphorical sense. And without that metaphysical baggage, there's nothing especially problematic about cross-temporal relations.)
(B) One might have a different concept of "harm" in mind. (But what, exactly? A rival analysis to my counterfactual account is demanded here.)
(C) Some of KTK's comments betray a commitment to presentism: the misguided view that only the present moment exists. Granted, there cannot be cross-temporal relations of harm (or anything else for that matter) if other times -- and their inhabitants -- do not exist. So much the worse for presentism.
(D) Ideological commitments regarding abortion might cloud one's judgment of related issues. This is a less charitable explanation, but one might be suspicious that KTK appears motivated to establish that it's okay for women to do anything they like to their fetuses. Or witness the gleeful accusations from pro-lifers that I needed to resort to "metaphysical gymnastics" in order to "justify [my] pro-abortion stance". Partisanship and wishful thinking blinds them to the more general nature of the problem.
To overcome at least this last problem, let me offer a thought experiment which cuts to the core of the issue. Suppose that babies grow on trees. Actually, they're not really babies (yet), but just baby-shaped pieces of wood: life-sized wooden statues. Real babies are made by a priestess asking God to turn
Here are some obvious facts about the situation, which absolutely everyone (no matter their views about real-life fetuses, abortion, or whatnot) ought to agree with:
1) Wooden statues do not have moral interests.
2) You can damage a wooden statue, say by sawing its limbs off.
3) Some wooden statues are "vitalized", or turned into actual persons, who inherit the damage.
4) Actual persons have interests, so the damage does harm them, and is hence morally bad.
5) So, it can be morally bad to damage a wooden statue, if the statue will be turned into an actual person, even though the statue itself lacks moral significance.
6) However, no harm is done by damaging a statue that will never be vitalized. (It's just wood, after all.)
So, there you go. No matter what you think of fetuses, everyone needs to follow my "metaphysical gymnastics" in order to make moral sense of the possibility of vitalizing wooden statues. Clearly, the early acts of damage can cause the future person's life to go worse than it otherwise would have. That is to say, it can harm them. It's no big deal that the person doesn't exist at the time of the damage. Things that happen before we exist can influence how well our lives go, and a negative influence on welfare is precisely the definition of "harm". So there's nothing especially mysterious about these "temporal acrobatics". Their occurrence is quite straightforward, and even to be expected.
I think everyone has to agree with what's been said so far. But there are more contentious issues at the intersection of time and welfare. Some of these are discussed in my old post 'Respecting Past Desires' (there's a really great discussion in the comments too). There I suggest that our welfare level at a time t depends on the desires we have at t being fulfilled. But if some of those t-desires are about past or future events, then those events will influence how well-off we are at t.* Indeed, I even hold that we can be harmed by events which occur after our deaths. As I once commented on another blog:
I think a person can be harmed even after they are no longer (presently) existing. We just need to understand the harms as retroactive: your present actions are making the earlier person worse off. We are better off when our desires are fulfilled, but it doesn't matter when they are fulfilled.
Suppose you dedicate your life to preserving ancient works of art. Then, after your death, someone burns down the gallery where all your preserved work was stored. They have made it so that your life went worse, since you failed in your primary goal. The exact timing of the failure doesn't matter.
I think that those who dismiss the wishes of the dead tend to have a crudely hedonistic conception of wellbeing. But as you rightly note, we often feel that we can be harmed without our knowing (e.g. by people on the other side of the world). Why should a separation in time be any more significant than one in space?
Desire-fulfillment theories of welfare hold, roughly, that what's good for us is to get what we most want. But some of the things we care about are not present. We care about the future, and even what happens after we die. My life goes better if my desires are fulfilled rather than thwarted. The future can influence the latter sorts of facts (about desire-fulfillment), and thereby also the former (about the quality of my life).
Again, I think there's nothing especially outrageous or surprising about any of this. Still, I grant that the view can at least coherently be denied. It isn't totally insane to deny that we can be harmed by events that occur after we cease to exist. On the other hand, I do think it's completely untenable to deny that we can be harmed by events that precede our existence.
* = (But does it really make sense to talk about momentary welfare values? I'm now more inclined to see welfare more holistically as the value of a whole life. Then, although harmful events all occur at some time or other, there is no particular time at which the harm is received by the person. Again, letting go of the misguided "physical transmission" analogy makes the temporal issues much more straightforward.)