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Picture a map of the United States. Each state is marked out in red or yellow or green or orange, in between the blue that is saved for the oceans and the dull white of Mexico and Canada. Now imagine a series of walls ten miles high running along the boundaries of each block of color and shutting every state off from the states around it. It will be helpful to remember that problems of interstate taxation are treated by the law very much as though those imaginary walls were real.
Now the average man knows well enough what a tax is. It is a sum of money that must be paid to a government because the payer owns something or does something or gets something. If the tax is "on" anything it is on the man or the company that must pay the tax. Sometimes the tax is a flat tax, like a two dollar school tax that must be paid by everyone who lives in the school district. More often the amount of the tax is measured according to the value of whatever the taxpayer owns or does or gets that makes him have to pay the tax. So much seems sufficiently simple.
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