Congress are not to lay taxes ad libitum, for any purpose they please; but only to pay the debts, or provide for the welfare, of the Union. In like manner, they are not to do any thing they please, to provide for the general welfare, but only to lay taxes for that purpose.
To consider the latter phrase, not as describing the purpose of the first, but as giving a distinct and independent power to do any act they please which might be for the good of the Union, would render all the preceding and subsequent enumerations of power completely useless. It would reduce the whole instrument to a single phrase that of instituting a Congress with power to do whatever would be for the good of the United States; and, as they would be the sole judges of the good or evil, it would be also a power to do whatever evil they pleased. -- Thomas Jefferson 1791