This is an interesting issue. Can Congress tax the gain in an asset's value? In other words, if your asset, whether stocks or real estate, have a gain in market value, even if you don't sell that asset and realize a gain, can Congress apply a tax to it? If so, it opens the door to a whole new class of taxes, not based on actual money received, such as in a sale of a stock or other piece of appreciated property, but rather, on the theoretical or market value of the asset. For example, can Congress tax you on the gain in the value of a stock you own even if you don't sell it?

https://news.yahoo.com/supreme-court-dec...36912.html

"The Constitution does not allow Congress to point at any pot of money and call it ‘income’ and then income-tax it,” said Andrew M. Grossman, the Moores’ lead counsel. “‘Income’ means the same thing now that it did when the 16th Amendment was ratified: gains that have been realized by the taxpayer."

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