Why you shouldn't invest in companies whose business you don't understand?

You shouldn't invest in companies whose business you don't understand because you won't be able to judge their future potential and vulnerabilities.

While intimacy with a company's business is not essential to making a decision to buy the stock, later "hold" or "sell" decisions may be clearer if you understand its competitive position in the marketplace, the value of its products or services, and the character of its industry's problems.  These are common-sense issues for which a simple understanding of the business will suffice.