An excerpt from Infoshop.org's Anarchist FAQ. This is an excerpt from the section on "Why is the history of capitalism important?" which I present here as a compelling argument in favor of slavery reparations.
Let us assume a ship sinks and 50 people get washed ashore on an island. One woman has foresight to take a knife from the ship and falls unconscious on the beach. A man comes along and steals her knife. When the woman awakes she cannot remember if she had managed to bring the knife ashore with her or not. The man maintains that he brought it with him and no one else saw anything. The survivors decide to split the island equally between them and work it separately, exchanging goods via barter.
However, the man with the knife has the advantage and soon carves himself a house and fields from the wilderness. Seeing that they need the knife and the tools created by the knife to go beyond mere existing, some of the other survivors hire themselves to the knife owner. Soon he is running a surplus of goods, including houses and equipment which he decides to hire out to others. This surplus is then used to tempt more and more of the other islanders to work for him, exchanging their land in return for the goods he provides. Soon he owns the whole island and never has to work again. His hut is well stocked and extremely luxurious. His workers face the option of following his orders or being fired (i.e. expelled from the island and so back into the water and certain death). Later, he dies and leaves his knife to his son. The woman whose knife it originally was had died long before, childless.
Note that the theft did not involve taking any land. All had equal access to it. It was the initial theft of the knife which provided the man with market power, an edge which allowed him to offer the others a choice between working by themselves or working for him. By working for him they did "benefit" in terms of increased material wealth (and also made the thief better off) but the accumulate impact of unequal exchanges turned them into the effective slaves of the thief.
Now, would it really be enough to turn the knife over to the whoever happened to be using it once the theft was discovered (perhaps the thief made a death-bed confession). Even if the woman who had originally taken it from the ship been alive, would the return of the knife really make up for the years of work the survivors had put in enriching the the thief or the "voluntary exchanges" which had resulted in the thief owning all the island? The equipment people use, the houses they live in and the food they eat are all the product of many hours of collective work. Does this mean that the transformation of nature which the knife allowed remain in the hands of the descendants of the thief or become the collective property of all? Would dividing it equally between all be fair? Not everyone worked equally hard to produce it. So we have a problem -- the result of the initial theft is far greater than the theft considered in isolation due to the productive nature of what was stolen.
In other words...when the property stolen is of a productive nature, the accumulative effect of its use is such as to affect all of society. Productive assets produce new property, new values, create a new balance of class forces, new income and wealth inequalities and so on. This is because of the dynamic nature of production and human life. When the theft is such that it creates accumulative effects after the initial act, it is hardly enough to say that it does not really matter any more. If a nobleman invests in a capitalist firm with the tribute he extracted from his peasants, then (once the firm starts doing well) sells the land to the peasants and uses that money to expand his capitalist holdings, does that really make everything all right? Does not the crime transmit with the cash? After all, the factory would not exist without the prior exploitation of the peasants.
Read the whole FAQ here. Available in 9 languages.