The document discusses the law of diminishing marginal utility. It states that as consumption of a good increases, the marginal utility (additional satisfaction) from each additional unit decreases. This is because wants are unlimited but individual wants can be satisfied. The marginal utility curve is downward sloping, showing diminishing returns to scale. An example is given of a thirsty man drinking glasses of water, where the utility declines with each additional glass. Exceptions to the law include rare goods and intoxicating substances.