In the piece I posted about here a couple of days ago, there's a reference in passing to 'the environmentalist maxim that we have only borrowed the world from our grandchildren'. I'm familiar with the sentiment, though I'd never come across the maxim. But I don't think it's a persuasive way of thinking about the responsibilities of the present generation with respect to the environment.
I take it, first of all, that 'grandchildren' is meant to refer generically to future generations, rather than to appeal to people's concern for their own particular blood descendants. I take it this way because otherwise the maxim waives the responsibility of anyone who will have no grandchildren, and it probably lets off far more people than that: everyone who'd be inclined to say that beyond their own proximate - their knowable - blood relations, they aren't going to feel as acutely bothered as for those. That may be rather a big exclusion, whereas I assume the maxim is aiming to persuade as many people as possible.
If, therefore, the people we are thought to have borrowed the world from are future people more broadly, then what is being asserted is that future people have rights against us in the matter of our management and use of the world's resources. If these are 'borrowed', then we hold them, at least in part, on loan; so that a property right in such resources is being asserted for future people against present people.
I don't think it makes sense to assert that future people, before they are actual people, have rights.
Here's why no putative future person can have rights. Until any such person comes to exist, s/he can't have a right to be brought into existence. There are various ways of seeing this. For you (and here I do mean you) to have had the right to come to exist, it would have needed a coercive obligation upon your biological parents to conceive you, with all that this involves in the way of getting together, exact timing and so forth. Not only is this not an obligation that could be supported as morally defensible, it is also a logically impossible one, since until you came to exist no one could have known who you were, to aim for so to speak, or how to do this aiming even assuming an impossible knowledge of your so far unformed identity. If no one can have a right even to exist before they do exist, it follows that they can't, before they exist, have any rights that depend upon their existing.
But perhaps a collectivity, future people of as-yet indeterminate individual identities, can jointly hold a right against presently-existing people. I don't think so. For the same style of reasoning as I've just undertaken for the individual case can be repeated for any putative future group. Any future they that you can hypothesize may never come to exist, and it therefore doesn't make sense to say of them that they had a right to be brought into existence - or the right to vote, to freedom of expression, to inherit a decently preserved environment, or what have you. At the limit, since one generation of human beings, this generation, could destroy all human life and with it the preconditions of any human future at all, the existence of all future generations is put in question and therewith the intelligibility of assigning them rights.
Finally, if the people of, say, 2254 have rights they could claim against us once they exist, then by the same token we would have rights against the people of 1760. Try asserting - try collecting on - such rights. It makes no sense.
My point, here, has not been to argue against there being duties to future generations - against a moral responsibilty on the part of existing people not to be environmentally reckless. It has only been to challenge the notion that such duties depend on some logically and morally prior rights of future people. Not all obligations depend on there being correlative rights. You may feel you have duties of generosity to others, without anyone specific having a right to make claims upon that quality of your conduct. People who deny that animals have rights can still think there are duties not to be cruel to them. I won't try to explore here what 'ecological duties' could be based on, but I think that the idea of the continuity of the human species plays an indispensable role in the way in which nearly all human beings construe the 'meaning of life', and that this fact will play a central role in any argument for such duties.