"But while intangible assets represent income which accrues out of certain immaterial relations between their owners and the industrial system, and while this income is accordingly not a return for mechanically productive work done, it still remains true, of course, that such income is drawn from the annual product of industry, and that its productive source is therefore the same as that of the returns on tangible assets. The material source of both is the same; and it is only that the basis on which the income is claimed is not the same for both. It is not a difference in respect of the ways and means by which they are created, but only in respect of the ways and means by which these two classes of income are intercepted and secured by the beneficiaries to whom they accrue. The returns on tangible assets are assumed to be a return for the productive use of the plant; returns on intangible assets are a return for the exercise of certain immaterial relations involved in the ownership and control of industry and trade."