How far we are to consider the interests of posterity when they seem to conflict with those of now-existing human beings? The answer to this, though,… - Henry Sidgwick

Each person is morally obliged to regard the good of anyone else as much as his own good, except when he judges it to be less, when impartially viewed, or less certainly knowable or attainable by him.

Is it total or average happiness that we seek to make a maximum?...we foresee as possible that an increase in [population] numbers will be accompanied by a decrease in average happiness...if we take Utilitarianism to prescribe, as the ultimate end of action, happiness on the whole...it would follow that, if the additional population enjoy on the whole positive happiness, we ought to weigh the amount of happiness gained by the extra number against the amount lost by the remainder. So that, strictly conceived, the point up to which, on Utilitarian principles, population ought to be encouraged to increase, is not that at which average happiness is the greatest possible...but that at which the product formed by multiplying the number of persons living into the amount of average happiness reaches its maximum.

I may begin by laying down as a principle that ‘all pain of human or rational beings is to be avoided’; and then afterwards may be led to enunciate the wider rule that ‘all pain is to be avoided’; it being made evident to me that the difference of rationality between two species of sentient beings is no ground for establishing a fundamental ethical distinction between their respective pains.