However careful we are to introduce changes gradually and to make each a small proportion of the whole, when eventually they add up to a considerable change we hesitate to attribute identity to such different objects. But we have a device through which we can induce the imagination to go one step further ·in attributing identity where really there is none·—namely, relating the parts to one another through some common end or purpose. A ship of which a considerable part has been changed by frequent repairs is still considered ‘the same’ even if the materials of which it is composed have come to be quite different. Through all the variations of the parts, they still serve the same common purpose; and that makes it easy for the imagination to move from the ship before the repairs to the ship after.
Tnh 1 4 06 10