Bill Gross, the co-chief investment officer and co-founder of the Pacific Investment Management Company, asserts that stock investing is "dead as we know it" in his August investment letter, which was published Tuesday.

PIMCO has $1.82 trillion in assets is the world's largest bond mutual fund.

"If financial assets no longer work for you at a rate far and above the rate of true wealth creation, then you must work longer for your money," Gross wrote.

Gross took particular issue with Wharton Finance Professor Jeremy Siegel's constant of 6.6% return from stocks, calling the past percentage return a "historical freak, a mutation likely never to be seen again as far as we mortals are concerned."

Earlier this month, Gross predicted real GDP growth at 1.5% for the next decade. Based on this assumption, investors should get used to smaller investment returns. Siegel took exception to Gross's comments on CNBC Tuesday, saying "Gross is basically being misleading with his comparison, because total return of the stock market is not appropriate to compare to GDP. You definitely can have a return greater than GDP growth. There is nothing uneconomic about it."

"The thing is that capital gives out dividends, it gives out interest, it gives out return. When you add that all together, it's going to be greater than GDP growth. Even in a non-growing economy, you have situations where return is greater than GDP growth."

The bond manager was not just criticizing the equity market but had equally harsh words for bond investing. "It is even more of a stretch to assume that long-term bonds -- and the bond market -- will replicate the performance of decades past," Gross wrote. In the current environment of low interest rates, investors should expect "mere survival" from an investment in bonds.  

Gross indicated that investors following momentum-chasing stocks and bonds based on a 30-year bull run are setting themselves up for a fall.

In his August letter Gross stated that the only "magic potion" policy-makers have to get higher returns is through a pro-inflationary environment. He noted that inflationary policy might work for bonds, but would have a negative impact on stocks.

"Unfair though it may be, an investor should continue to expect an attempted inflationary solution in all almost all developed economies over the next few years and even decades," Gross wrote. "The cult of equity may be dying, but the cult of inflation may have only just begun."

Is it possible we are headed for another bull run? Probably not, considering all the uncertainty throughout Europe and in the U.S. According to data compiled by Bloomberg, PIMCO's Total Return Fund gained 7.3% during the past year. The gain beat 73% of its peers.