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Since the earnings that make up the taxed income are based on the structure of state, society and economy that taxation upholds, the use of taxation is not a usurpation, but rather a reclamation. So far as the state represents the people, taxation is not theft.
Robbery or theft implies that you don't get anything in return for it. However taxes are used to provide for a wide variety of public services that you get in return such as highway maintenance, schools, healthcare (outside the US at least) and emergency services.
Robbery is the illegal taking of property, in the presence of the owner, against the owner's will, by violence or intimidation. Taxation is the legal duty of owners to surrender a portion of their money to the government to sustain its operation.
They have similarities but are not identical.
Robbery is when something of value is taken from you with no expectation of return or replacement. Taxation is like a business transaction. You pay taxes with the expectation that, that money will be used to improve infrastructure, protect you, or pay the cost of keeping a country running.
Taxation is extortion because there is no possibility to participate in modern society without paying taxes.
Taxation is voluntary to the extent that people are unwilling to suffer the consequences of a tax revolt.
There is little preventing a state from funding non-essential programs voluntarily as a trial run. However the power is still in the hands of the citizens.
If "taxation" is split into 2 phrases we might agree more: "Taxation to pay for infrastructure" vs "Taxation to pay for political overindulgence". Most people would be happy to pay over money for the former but not for the latter thereby making the latter robbery and the former not.
Taxation was and has always been a source of revenue that the government needs to function. It was how during the American revolution money was raised after defeating Britain, otherwise making ends meet with the debts we were in would have been nigh impossible.
If robbery is to take property by force, this is true. However, if someone were to reclaim property that rightfully belongs to them by force, that would also be robbery (though just). Robbery has unjust connotations, so this definition may not be accurate, or robbery may sometimes be justified.