(b) Taxes may be laid upon _incomes_, as they come into the possession of the owner. Usually, only monetary incomes that arise in commercial transactions are taxable, and no attempt is made to estimate the value of psychic incomes. Commercial incomes are more easily measured, but the omission of the other elements must cause many inequalities in the burden of the tax as between two individuals controlling equal incomes of real things. (c) Taxes may be on _property_, either general upon all property in the taxing district, or special, upon certain forms of property. A property tax may be specific or _ad valorem_, in proportion to value, as to the method of its determination. Since the value of material wealth is the capitalization of the rentals at the prevailing rate of interest, a general, _ad valorem_, property tax, so far as it applies to material wealth, and if it were accurately assessed, would take an approximately equal proportion of wealth-incomes. It does not, of course, touch directly incomes derived from wages and salaries, but it reduces their purchasing power in many cases. It is in some respects more searching than a tax on actual rents, for it reaches the prospective, or speculative, rental. (d) Taxes may be on _expenditure_ (sometimes called taxes on consumption). This is but another mode of attacking income, for in the long run most income is spent, not always by the individual who earned it, but by some one, and thus it is reached by a tax on expenditure. Usually in the United States the tariff duties are accounted to be taxes on expenditure, as also the internal revenues (also called excises) of the national government. In time of war, internal revenues are extended in the United States to a multitude of articles, but usually they have been limited (with minor exceptions) to liquor and tobacco. Most of these taxes are in fact levied not at the time of purchase by the ultimate consumer, but upon the specific goods in the hands of some merchant or business agency, and some of them are essentially special property taxes and others are business taxes of the kind next to be mentioned. (e) Taxes may be levied on selected agencies of industry or on the process of _business_; such are business taxes, licenses, taxes on investment in business, and corporation taxes. These burdens are diffused and rest eventually on some income, rarely to be ascertained exactly.