Fair Share

Yesterday, President Biden said, "I will not impose any tax increase on people making less than $400,000. But it’s time for corporate America and the wealthiest 1 percent of Americans to just begin to pay their fair share....But I will not add a tax burden, additional tax burden on the middle class of this country. They’re already paying enough."

According to the nonpartisan Tax Policy Center, the middle class (defined here as the middle quintile of the income distribution) now pays about 13 percent of its income in federal taxes. The top 1 percent pays about 30 percent of its income in federal taxes.

I wonder: What constitutes a "fair share" in President Biden's eyes? On what basis does he conclude that the current distribution of the tax burden is not fair?