Howard Marks has some harsh words for the bitcoin community and the rise of digital currencies, which have become all the rage lately.

“Digital currencies are nothing but an unfounded fad,” said the co-chairman of Oaktree Capital Management, who was among the first to sound the alarm on the 2008 financial crisis.

Bitcoin BTCUSD, -0.18% is “based on a willingness to ascribe value to something that has little or none beyond what people will pay for it,” wrote Marks in his latest memo to clients.

Read: Bitcoin isn’t real, and markets are darn hot, warns Howard Marks

The billionaire investor believes an increasing lack of faith in fiat currencies on top of millennials’ love of “all things virtual” have allowed cryptocurrencies to emerge as a potential investment tool.

“Maybe I’m just a dinosaur, too technologically backward to appreciate the greatness of digital currency. But it is my firm view that the ability of these things to gain acceptance is just one more proof of the prevalence today of financial naiveté, willing risk-taking and wishful thinking,” he said.

Marks boasts a net worth of $2 billion, according to Forbes. His investing letters are often parsed by market participants for insightful nuggets.

He freely admits he is stumped by cryptocurrencies and says he isn’t alone among savvy Wall Street investors puzzled by its rise.

“But they are not real!!!!! Nobody has been able to make sense to me of these currencies,” he said.

One of the biggest pitfalls of bitcoin and its digital peers, according to Marks, is that they are mostly used to buy other “imaginary” money or to invest in companies that will create other new currencies.

Howard Marks

But regardless of whether bitcoin is real or not, the more important question posed by Marks is whether the digital currency is suitable for investment or if it should be treated as just another tool for speculation.

“Serious investing consists of buying things because the price is attractive relative to intrinsic value,” explained Marks. “Speculation, on the other hand, occurs when people buy something without any consideration of its underlying value or the appropriateness of its price, solely because they think others will pay more for it in the future.”

Related:What are ICOs and why is the SEC taking steps to protect investors from them?

Francisco Blanch, head of global commodities and derivatives research at Bank of America Merrill Lynch, earlier this week also stressed that for all the buzz generated by bitcoin, it still has some ways to go before it can be considered a legal tender.

“Most regulated financial institutions allow their clients to borrow against financial or physical assets, but we are not aware of any major institution that takes cryptocurrency as collateral at the moment. Thus, in our view, a key step for bitcoin would be for it to become pledgeable collateral,” he said.”

Treasury Secretary Steven Mnuchin said cracking down on illicit activity in cybercurrencies is a focus of regulators. “I share your concerns about bitcoin and others and them being used for illicit activity, said Mnuchin during congressional testimony on Thursday, referring to the recent arrest of a Russian man accused of laundering some $4 billion using bitcoins.

Meanwhile, prominent financial adviser Josh Brown, who proclaims to idolize Marks, had a bit of an existential take on the whole cryptocurrency debate.

He agrees with Marks that bitcoin isn’t real and its value is based on nothing.

“But so what?” he asks. “Nothing is real, in truth.”

He goes on to argue that anything that doesn’t appear in nature may not be real—even countries, religion, and currencies.

Read:Bitcoin may have reached a tipping point, now that ‘Downtown’ Josh Brown just invested

“Things become real enough when people begin to believe and agree that they are. The Declaration of Independence didn’t make America real, it made people believe that it could be real and then will it into existence with their cooperation and their actions,” said Brown.