This section of the book is from the "Introduction To Public Finance" book, by Carl Copping Plehn.
The form of the income tax will be determined by the place given it in the system of taxation. If it were possible to administer a single tax of any sort in accord with the demands of justice, the income tax would be, theoretically, the one to be chosen. But the objections to any single tax, already stated, bear upon this as well as upon any other. Theoretically, it is best to make the income tax the central one of the system, the gaps of which are filled in by other taxes. If this be the intention, then the income tax can be arranged in the form in which it is most easy to administer. Thus the very small incomes can be exempt from the income tax, being covered by direct and indirect consumption taxes. In this way one source of difficulty and friction is avoided. Then no distinction need be made in the assessment of income from different sources. For if it be decided to tax income from funded investments at a higher rate than other forms of income, this additional tax can be laid on in the form of a property tax. How far the exemption of smaller incomes should go, or to what extent funded incomes should be more heavily burdened, depends upon the concrete facts in each case. An abatement of the burden in cases where there are already more than the usual claims on the income, as of a large family, is, also, sometimes given.