That assumption is to make time the only difference between the situations, because the point is that the total amount of utility over my life stays constant. If I lose utility during the time of the agreement, then I would accept a rate that earns me back an amount equal to the value I lost. But if I only "want" to use it today and I could use it to get an equal amount of utility in 3 months, then I don't have a preference.
because the point is that the total amount of utility over my life stays constant.
I don't think this is a useful approach. A major component of time preference is the fact that future is uncertain.
If you set up the situation such that there is basically no difference between the future and the present (both are certain, known, you yourself don't change, etc.) then yes, reshuffling utility between two otherwise identical points on a timeline is something you could well be indifferent to. However that's very far away from the real life and I thought we were talking about something with practical applications rather than idealized abstractions.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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