This section of the book is from the "Introduction To Public Finance" book, by Carl Copping Plehn.
Considerable confusion in the discussions of the different modes of taxation is due to the failure to distinguish clearly between the justification of taxation in general (i.e. why there should be any taxes at all), and the measure of taxation (i.e. whatshould be the basis upon which to decide how much each citizen should pay). The universally accepted justification of taxation is the common benefit conferred upon the individuals by the action of the government. But the common benefit is, strictly speaking, equal, while the taxed citizens are unequal in wealth. Therefore recourse has to be had to some other measure of taxation. - It lay nearest, in the search for such a measure, to assume that there was a difference in the benefit enjoyed by the different citizens. Thus one theory assumes that protection to life, liberty, and property is the chief benefit conferred, and that this benefit, or at allevents its cost, varies as the property varies, generally in exactly the same proportion. This theory has been called the benefit theory of taxation, because it attempts to estimate by the benefit conferred the amount of tax each individual should pay. The difficulties involved in measuring benefit, with sufficient accuracy to serve as a basis for taxation, led another school of thinkers to abandon that entirely. These writers felt that each citizen was necessarily a part of the organism of the State, one of the nourishing cells as it were. And as in all organisms of nature each organ or each cell contributes to the life of the whole, in accordance with its powers or strength, so each citizen should contribute as he is able. They claim that it is easier to measure ability than it is to measure benefit. This theory is called the faculty theory, the term "faculty" having been found in this sense in early tax laws. Generally speaking, this ability is supposed to be indicated in some way by wealth. But the advocates of faculty as a measure of taxation encounter a serious difficulty in attempting to ascertain whether faculty is proportional to wealth or increases more rapidly as wealth increases. A negative side of the same idea is presented when it is claimed that the tax should impose an equal sacrifice upon every citizen. In determining what constitutes equal sacrifice, we encounter the same difficulty as in determining how to measure ability.1
1 See Chap. III. for further discussion of this point. A full and instructive discussion of these theories is to be found in Seligman's Progressive Taxation in Theory and Practice.