The aims of taxation, the accomplishment of which shifting may aid or defeat, have been stated in the form of canons. The oldest and most famous are the four of Adam Smith.1 The fourth of these famous canons bears directly upon the problem in hand.

1 Wealth of Nations, Bk. V., Chapter II, Part II

Shifting adds to the burden without adding to the revenue coming into the treasury. Sometimes,however, by assessing the tax upon a part of a great economicprocess and permitting of a certain shifting, the tax is made more productive to the treasury. Now it is clear that the first duty of the fiscal officer is to fill the treasury as quickly and easily as possible. He has, therefore, no right to pass over a tax that is productive, in favour of one that is not, simply because the one is shifted and the other is not. If, however, it can be seen that the incidence is such as to entirely derange the system adopted and defeat the general aims, an attempt must be made to prevent it. If a tax can be found that is productive and at the same time is not shifted, it will be preferred to one that is shifted.

It follows directly from the fact that a tax should be as " productive " as possible, that those taxes chosen should have as little repressive effect as possible.1 For, to destroy the phenomenon is to lose the return.