CNBC's Jim Cramer knows that sentiment matters when stocks are constantly being driven higher by a raging bull market.
But the "Mad Money" host also knows that sentiment is extremely difficult to measure. Financial newsletters, which often lean bullish or bearish, are falling out of style. Mutual fund data offers an empirical look, but can be incomplete or disappointing.
"[Mutual fund inflows] remain pretty dismal, [with] more money coming out in the last month than going in despite the stunning gains we've seen," Cramer said. "Many weeks still produce big outflows from stocks, good weeks where people make money. Call that negative sentiment for certain."
Then there's anecdotal evidence. Cramer often gathers this from roundtables and conferences he attends. Lately, people seem to be getting more interested in the stock market (not committed, but interested, he noted).
"And you know what? I like that. I think the lack of warmth or love or even nodding acceptance of stocks as something that can make you money is downright wonderful for the bulls. See, if investors were all in, I would have to be very concerned," the "Mad Money" host said. "When everybody already loves stocks, you've got no tinder to start the fire."
The lack of enthusiasm suggests that there's still money on the sidelines that hasn't entered the market and, in Cramer's view, could be the key the bull market's next leg higher.
In the stock market's heyday, Cramer said he couldn't walk 5 feet down Wall Street without being asked about a stock.
Nowadays, people may ask him for photos or what he thinks about the unregulated cryptocurrency bitcoin, but very few ask him about individual stocks.
(Regarding bitcoin, Cramer usually points them to a 2012 episode of The Good Wife in which he vouched for bitcoin as a guest star in the courtroom.)
So Cramer has started asking people why they don't like the market. So far, he's been met with a cacophony of eye-opening answers.
"I'd say about a third of them tell me, 'I'm just not interested because it's too dangerous,'" the "Mad Money" host said.
Most of those people told Cramer that they lost everything in the 2008 financial crisis or the 2000 dotcom crash and would rather keep cash than invest in stocks.
"[It's] pretty amazing given how poorly cash has done versus stocks. The risk aversion is tremendous," he said.
Cramer has also heard from people who have been waiting for ages for a pullback and say they "missed the rally." A third group told Cramer that they didn't invest because of politics.
The fourth reason Cramer has heard is that people can't afford to invest in the stock market, despite Cramer's advice to start with small investments and build on their gains.
"The belief that only the rich own stocks and make money in them because the market's rigged against the little guy is a pretty constant refrain," the "Mad Money" host said.
All in all, Cramer has not ignored the trajectory of this rally. With the exception of the drug stocks, the consumer foods plays and the retailers, the bull market has been surging. But he has noticed that the sentiment doesn't match the gains.
"I believe that as long as people are incredulous or disdainful or scared of this market ... then there's trillions of dollars of tinder on the sidelines that can come in and take stocks higher," Cramer concluded. "And that process is pretty self-fulfilling; as they take stocks up, their old memories of the bad times disappear [and] new ones are created based on a better economy and a realization that the stock market, and even the most obvious of companies as plain as the nose on my face, can make you a heck of a lot more money than any other asset class under the sun."