When any guy offers you a chance to earn lots of money without risk, don’t listen to the rest of his sentence. Follow this and you’ll save yourself a lot of money.
Wide diversification is only required when investors do not understand what they are doing.
The investor’s chief problem—and even his worst enemy—is likely to be himself.
In the current up-cycle, over-weighted, liquid, large-cap stocks have benefitted from forced buying on the part of passive vehicles [e.g, index funds], which don't have the option to refrain from buying a stock just because it's overpriced.
The most important quality for an investor is temperament, not intellect.