The Wealth of Nations by Part 5 Chapter 2 Page 99

There are, however, two different circumstances which render the interest of money a much less proper subject of direct taxation than the rent of land.

First, the quantity and value of the land which any man possesses can never be a secret, and can always be ascertained with great exactness. But the whole amount of the capital stock which he possesses is almost always a secret, and can scarce ever be ascertained with tolerable exactness. It is liable, besides, to almost continual variations. A year seldom passes away, frequently not a month, sometimes scarce a single day, in which it does not rise or fall more or less.

An inquisition into every man’s private circumstances, and an inquisition which, in order to accommodate the tax to them, watched over all the fluctuations of his