The Dow Jones Industrial Average has established itself north of 12,000. The S&P 500 index is above 1,400. Those are close to record highs for both. Compare that to where they were just three years ago and it seems like the stock market is booming.

So are you going to invest now or sit this one out? Here are three myths that could be holding you back.

Myth No. 1: Prices can only go down from here
No one can predict the future accurately. In 1929, Irving Fisher, professor of economics at Yale University, tried when he said "Stocks have reached what looks like a permanently high plateau." So did an unknown Boeing engineer, who said, "There will never be a bigger plane built," after the first flight of the 247, which held only 10 people. Even when they are experts in the field, people have a very poor track record of predicting the future.

So, if you are worried that stock prices can only go down from here, invest. Twenty years from now, you'll be glad you did.

Myth No. 2: The wrong time to be in "the market"
Daily, we hear news stories about how the market is up or the market is down or the market is fearful about higher oil prices. Forget that. The "market" isn't alive and it has no single reason for the way it seems to act. The "market" is nothing more than the combined trading of thousands of different people, including you and me, making individual decisions about buying or selling.

Think about it this way: If an investor in Ford (NYSE:F) believes that higher oil prices will lower the demand for cars, he would sell his shares. Another investor with the same information might argue that higher oil prices will lead to higher profits for gasoline companies. She'd go long shares of ExxonMobil (NYSE:XOM). How can both of these investors be "fearful about higher oil prices" at the same time? And, further, how could the fear of higher oil prices concern a potential investor of $87 billion biotech firm Genentech (NYSE:DNA)?

In every kind of market condition, there are many different opportunities. The Foolish investor tries to find those opportunities, rather than subscribing to general statements about the "market."

Myth No. 3: Nothing left to invest in
True, it seems like indexes are setting daily highs. But that doesn't mean every stock is at a super high price. Remember, those are just two indexes, trying to represent what is happening in a much bigger arena. The Dow is made up of only 30 companies. Just because it is at record highs doesn't mean each member company is. Caterpillar (NYSE:CAT), for instance, is down 20% from earlier this year. Intel (NASDAQ:INTC) is down almost as much, despite its recent rise.

There are more than 8,000 publicly traded companies that are not represented by the Dow or the S&P 500. Even the broad market Russell indexes cannot accurately reflect the movement of every single company. Surely at least some of them would make good investments, regardless of what the indexes might be doing. For instance, back in November 1991, you could have invested in General Dynamics (NYSE:GD), held on through 1999 and late 2002 to early 2003, and today had a 2,400% return. That's an average of 23.6% per year. The Dow? It was down that month. So was the S&P 500.

You should tune out the noise and pay little attention to "the market" or the popular indexes. Instead, focus on finding the quality companies that you can buy to hold for the long haul. Oil up? The Dow down? Not a problem when your focus is elsewhere.

That's what Fool co-founders David and Tom Gardner do at Motley Fool Stock Advisor. Avoiding the three myths and focusing on superior businesses trading at reasonable prices has led them to some pretty impressive returns: their Stock Advisor picks are beating "the market" by more than 45 percentage points since inception in 2002. Give the newsletter a try free for a month and see how they can help you ignore these and other investing myths.

Fool contributor Jim Mueller loved reading Greek and Roman mythology when he was growing up. Maybe that helps him recognize other myths today. (Then again, maybe not.) He owns shares in Intel, an Inside Value recommendation, but no shares of any other companies mentioned. The Fool'sdisclosure policyis certainly no myth.