“I don’t know.” Those may be the three most important words for an investor. (Four if you want to picky.) The literature on behavioral finance has improved our knowledge of investor behavior over the past decade.

One common finding is that investors, and people in general, are overconfident in their abilities. Whether the question pertains to answering trivial facts, our driving abilities, or our stock-picking skills, if you ask a sample of people a sizeable majority of them will claim to be above average.

Rather than looking at “I don’t know,” as an admission of ignorance, investors should look at it as a sign of humility. There are too many stocks, bonds, funds, and ETFs for any one to know enough about some of them, let alone all of them. Warren Buffett came under some criticism during the Internet mania for not investing in technology stocks, let alone Internet stocks. Buffett viewed technology stocks to be outside of his core investing competence. In short, there is no shame in saying those three words.

This point was driven home this week when reading Barron’s this weekend. Sandra Ward interviewed Charles de Vaulx manager of the First Eagle Global Fund (SGENX). de Vaulx has a classic comment you do not often hear from investment managers.

What is your favorite investment now?

There is nothing. (emphasis added) When I look at the top 10 holdings of the fund, there are pretty good companies, businesses with a scope for some intrinsic value to grow over time, companies that have superb balance sheets so they are not about to go bankrupt, but there is nothing dirt-cheap that we are totally excited about.

No forced quips. de Vaulx feels comfortable saying there are no compelling buys out there. One group that is never at a loss for something to say are investment strategists. Michael Santoli in Barron’s surveys Wall Street’s investment strategists and finds no shortage of opinions. While some of these strategists have something interesting to say about the economy and stock market, the simple act of having to constantly forecast the market puts them on shaky ground.

Excellent investors are constantly questioning the underlying assumptions behind their investments. Some times that involves deliberately seeking out contrasting opinions. This may seem like simply seeking out another set of forecasters, but the point is a subtle one. Opinions without some sort of factual or statistical basis are indeed useless. However an intentional search for variant information can help one become a stronger, and more humble investor.

David Ranson and Penny Russell in Barron’s put forth seven opinions that are at utterly at odds with conventional wisdom. Their opinions on inflation, the dollar, the stock market, the housing market and oil prices could all be wrong, but if it helps test your investment theses then it may be worth a look.

Jonathan Clements in the Wall Street Journal is a solid writer on personal finance. The beginning of his piece on “Five Pieces of Conventional Wisdom to Ignore” hits this theme right on the head:

Question everything. (emphasis added)

The longer I write this column, the less faith I have in conventional financial wisdom.

We all rely on rules of thumb and generally accepted notions because we don’t have time to research every financial issue and probe every assumption. Life is just way too short.

Yet conventional financial wisdom often doesn’t stand up to close scrutiny. Here are five ideas I have tossed out in recent years.

The five items are worth reading as well.

In short, question conventional wisdom. Just because somebody smart says something, does not mean it is true. Wise investors are constantly questioning the world around them. Some times we will not have enough of, or the right kind of, information needed to make an informed investment decision. There is no shame in saying, “I don’t know.” Shame comes from acting on limited information, useless forecasts or outdated conventional wisdom.