St. Augustine is widely quoted as having said, “Dear Lord, give me chastity, but not yet.” House Republicans seem to feel the same way about new revenues.

House Speaker John Boehner (Alex Wong/Getty Images)

Yes, it’s true that the letter to President Obama on Monday from House Speaker John Boehner and other House Republican leaders says that they are now willing to accept the need for new revenue. They propose to raise $800 billion over a decade through tax reform – and claim they do this and cut rates at the same time.

But they appear to want to work out this tax reform next year. And, in any event, what they do not do in their letter is lay out exactly which deductions they would eliminate or limit, or what mechanism they would use. On taxes, this is not an offer. It’s a promise. (They’re also rather vague about many of their budget cuts.)

The truth is that tax reform simply cannot raise the revenue needed for a deal. Going back to the Clinton rates on upper-income Americans can. But it’s perfectly fair to talk about alternative ways to raise revenue — as long as politicians explain exactly how they would do it. They don’t call it the Ways and Means Committee for nothing. The Republicans’ letter is short on both ways and means.

Moreover, when President Obama early in his term proposed very modest limits on deductions for upper-income Americans, Republicans screamed and yelled and charged the president with trying to undermine our great not-for-profit institutions by limiting the charitable deduction. Have Republicans changed their mind on this? We don’t know because they haven’t told us. And if they do not eliminate the deduction for charitable giving, it becomes even more difficult for them to raise all the money they claim they can.

You can be sympathetic to President Obama’s opening offer, as I am, or you can be critical of it. This is possible because he put out details of what he has in mind. There is no reason to take House Republicans seriously on their pledge to raise revenue until they explain, in detail, where the new revenue will come from – and until they say flatly that they are willing to pass their proposal by the end of the year.