Howard Marks, Co-Chairman, Oaktree Capital. David A. Grogan | CNBC

Billionaire investor and Oaktree Capital Management Co-Chairman Howard Marks is worried to hear investors say "this time it's different" or openly wonder if the historic bull market and economic success "can only get better forever." In a 12-page letter sent to Oaktree clients on Wednesday, Marks questioned nine financial theories he's heard in recent meetings, including the notion that central bank policy can lead to evergreen market success and that economic recessions can be consistently delayed. Here is the full list of hypotheses Marks scrutinized as written in his letter: There doesn't have to be a recession. Continuous quantitative easing can lead to permanent prosperity. Federal deficits can grow substantially larger without becoming problematic. National debt isn't worrisome. We can have economic strength without inflation. Interest rates can remain "lower for longer." The inverted yield curve needn't have negative implications. Companies and stocks can thrive even in the absence of profits. Growth investing can continue to outperform value investing in perpetuity. "The nine propositions reviewed above all represent variations on 'things can only get better forever,'" Marks wrote. "If they're the ideas guiding investors today, that should be considered worrisome." Though it's always difficult to predict the timing of an economic downturn, Marks said that he's always been confident that a recession is on the horizon at some point.

"We've always had economic cycles, and I believe we always will," he wrote. "Eventually, favorable developments will lead people to engage in behavior premised on excessively optimistic assumptions, and eventually the over-optimism of those assumptions will be exposed and the excesses will correct in a period of negative growth." "Very soon, the current recovery is bound to become the longest in U.S. history," he continued. "However, I believe the odds are that it's closer to the end than the beginning. ... The recovery is likely to go on longer, but perhaps not much longer." Marks, known for his prescient investment calls, correctly warned about the 2008 financial crisis and the dot-com bubble implosion. Oaktree Capital had $119 billion of assets under management as of March. Marks has a net worth of $2.1 billion, according to Forbes.

Do we want the Fed preventing recessions?