The key to investing is not assessing how much an industry is going to affect society, or how much it will grow, but rather determining the competitive advantage of any given company and, above all, the durability of that advantage.
Investors making purchases in an overheated market need to recognize that it may often take an extended period for the value of even an outstanding company to catch up with the price they paid.
If you know how to value businesses, it's crazy to own 50 stocks or 40 stocks or 30 stocks, probably because there aren't that many wonderful businesses understandable to a single human being in all likelihood. To forego buying more of some super-wonderful business and instead put your money into #30 or #35 on your list of attractiveness just strikes Charlie and me as madness.