What can one learn from income tax?

Should income be taxed? Even when we see tax as the only way of income for the government, I find no justification at all, for income tax. Especially since a government can choose between many kinds of taxes. It looks economically illogical that we tax the productive output of a society. Taxing anything means you will get less of it. Which means, by taxing the income, each one earns from one's efforts, we are making ourselves 'progress' towards a less productive society. At least, the productivity will be much less then what it would have been, had income not been taxed. Why have we been ignoring this, since long?
Also, why did't we start to tax expenditure? The rich, who spends more, would have been taxed also, more.
Hence income tax is one more instance, which proves that our natural propensity is to the irrational.