Here’s an example. Suppose you have a cup of water that’s gradually evaporating. It has 100 grams of water now, and it’s losing 10 grams per hour. How much water did it have 1 hour ago?
One way of expressing that mathematically is 100 – (-10). And I think it makes intuitive sense that if the cup is losing water over time, then in the past it had more water than it has now.
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