Oil prices jumped Thursday to more than $50 a barrel on news of Saudi Arabia's attack in Yemen, and that doesn't bode well for stocks, says Dennis Gartman, publisher of The Gartman Letter.

Oil prices have "probably bottomed" at $42.03, the six-year low reached March 18, he tells CNBC.

"If we're seeing this sort of activity in the crude oil market, this sort of rapid rise higher predicated upon the problems in the Mideast, [it] does make it more difficult to be bullish on stocks," Gartman argues.

"It's probably time to be less bullish. It's probably time to be less involved. It's probably time to reduce one's exposure a bit."

The S&P 500 index has dropped this week, but remains less than 3 percent below its March 2 record high.

"We have some problems here," Gartman explains. "We've seen far too little leadership on the upside [and] far too much parabolic toppy-type action in [biotechnology and pharmaceutical stocks]. That sort of activity bothers me greatly."

"All things being equal, a weak dollar begets stronger commodity prices. So yes, if crude oil has seen its lows, the odds are better that the dollar has seen its highs. But I'm not quite willing to make that statement yet."

But he doesn't believe stocks' six-year rally is over.

Meanwhile, New York Post columnist John Crudele sees the stock market as rigged. "When I started making that claim years ago — and provided solid evidence — people scoffed. Some called it a conspiracy theory, tinfoil hats and that sort of stuff. Most people just ignored me," he wrote.

"But that's not happening anymore. The dirty secret is out."

Lewis' theory is based on the impact of high-frequency trading. Crudele sees it a bit differently. He noted that economist Edward Yardeni sees Fed easing as rigging the market for gains.

In addition, "The Bank of Japan — and other central bankers around the world — could easily be purchasing shares of American companies to help out the US stock market," Crudele says.

"The rigging of U.S. stock markets by foreign entities has likely been going on for some time."

Finally, companies themselves are rigging the market through massive share buybacks, he maintains.

By reducing the number of shares outstanding, "this accounting trick boosts the calculation of profit-per-shares because the numerator of the equation (earnings) remains the same while the denominator (outstanding shares) is reduced."
 

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