Wharton professor and author Jeremy Siegel says that we’re in a “stealth” bull market that is far from over.

“People say the market is ahead of itself? It is nowhere near ahead of itself,” Siegel said at a recent conference, Reuters reports. Siegel, whose research on stock market returns goes back to the early 1800s, says the S&P 500 is currently about 20% below its long-term trendline.

Siegel, who is leery of Treasury bonds, said stocks are historically cheap. In years when interest rates were lower than 8%, he said, the S&P 500 has traded for about 19 times earnings. “Based on the estimated 2011 earnings, for the S&P 500 and a valuation multiple of 15, stocks have 13 percent upside, he said,” according to Reuters. “Stocks could rise 43 percent if investors assume a P/E multiple of 19 times.” Siegel is particularly high on high-yielding stocks, Reuters adds.