How Do You Cap A Tax Cut?

Seriously.

Yesterday, according to the WaPo, the Senate voted to halve the President’s tax cut proposal. Has anyone seen the actual wording of the amendment?

I ask because the government really has no power to increase or decrease tax revenues with any precision. All it can do is prescribe rates. It’s like a video game controller that can change velocity, but can’t control the cursor position directly. All estimates of revenue from tax rate changes are just that, and the discussion about the size of a tax cut is a fiction, because in reality no one knows what the true implications of a rate change will be–it’s too dependent on things like the future state of the economy and changes in peoples’ behavior that might result from the change.

So, just how did the Senate propose to limit the tax cut to a specific dollar amount? Was it like declaring pi to be three by fiat?