Books updating the classification systems used by many libraries are not free -- in fact they are very expensive. The only way to sell copies of them is to make the potential buyers believe they need to adopt the most recent system. Thus, these frequent changes in the classification systems are just a ploy by the publishers to make libraries buy their products. The reasoning above is most vulnerable to criticism because it ['concludes that there is no need ever to change classification systems', 'concludes that a possible ulterior motive must be the only motive', 'fails to consider that there may be potential buyers of these books other than libraries', 'fails to consider that the libraries cannot afford to buy every book they want']