"Land monopoly is not the only monopoly, but it is by far the greatest of monopolies -- it is a perpetual monopoly, and it is the mother of all other forms of monopoly. Unearned increments in land are not the only form of unearned or undeserved profit, but they are the principal form of unearned increment, and they are derived from processes which are not merely not beneficial, but positively detrimental to the general public."