From Margin of Safety:
Business valuation is a complex process yielding imprecise and uncertain results. Many businesses are so diverse or difficult to understand that they simply cannot be valued. Some investors willingly voyage into the unknown and buy into such businesses, impatient with the discipline required by value investing. Investors must remember that they need not swing at every pitch to do well over time; indeed, selectivity undoubtedly improves an investor’s results. For every business that cannot be valued, there are many others that can. Investors who confine themselves to what they know, as difficult as that may be, have a considerable advantage over everyone else.