Donald Trump sued Timothy O’Brien, author of TrumpNation: The Art of Being the Donald (2005), and his publisher, for $5 Billion for allegedly underestimating his net worth.

Trump’s suit was dismissed in 2009, but O’Brien did get to see (at least, heavily redacted versions of) Donald Trump’s tax returns. With Donald Trump running for president, Timothy O’Brien thinks you ought to get to see Trump’s tax returns, too, and he explains why.

1) Income: Trump has made the size of his fortune a centerpiece of his presidential campaign, implying that it’s a measure of his success as a businessman. He has also correctly noted that the income shown on his tax returns isn’t a reflection of his total wealth. Even so, income is a basis for assessing some of the foundations of any individual’s wealth — and would certainly reflect the financial wherewithal of the businesses in which Trump is involved.

After Fortune’s Shawn Tully dug into Trump’s financial disclosures with the Federal Election Commission and an accompanying personal balance sheet his campaign released, he noted in March that Trump “appears to have overstated his income, by a lot, which could be the reason he has so far tried to avoid releasing his returns.” Tully said that Trump apparently boosted his income in the documents by conflating his various businesses’ revenue with his personal income. Trump didn’t respond to Tully’s assessment, but he could clear up all of that by releasing his tax returns.

2) Business Activities: Trump has long claimed that his company, the Trump Organization, employs thousands of people. He has also criticized Fortune 500 companies for operating businesses overseas at the expense of jobs for U.S. workers. Trump’s returns would show how active he and his businesses are globally — and would help substantiate the actual size and scope of his operation.

3) Charitable Giving: Trump has said that he’s a generous benefactor to a variety of causes — especially war veterans — even though it’s been hard to find concrete evidence to support the assertion. Other examples of major philanthropic largess from Trump have also been elusive. Trump could release his tax returns and put the matter to rest.

4) Tax Planning: There’s been global attention focused on the issue of how politicians and the wealthy use tax havens and shell companies to possibly hide parts of their fortunes from authorities. If released, Trump’s returns would make clear whether or not he used such vehicles.

5) Transparency and Accountability: Trump is seeking the most powerful office in the world. Some of the potential conflicts of interest or financial pressures that may arise if he reaches the White House would get an early airing in a release of his tax returns.

For the last 40 years, presidential candidates have released their returns. Trump, of course, has portrayed himself as the un-candidate, the guy who bucks convention. But disclosing tax returns is a valuable political tradition that’s well worth preserving.

Donald Trump’s tax returns speak directly to issues of the candidate’s character and veracity, and obviously need to be released. Mitt Romney is right: If Trump does not release those tax returns before the GOP Convention, the Party ought to declare him ineligible for its nomination and the Convention Rules Committee should declare all Trump delegates released for the first ballot to vote for an eligible nominee.