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.
A prediction about the direction of the stock market tells you nothing about where stocks are headed, but a whole lot about the person doing the predicting.
The best business returns are usually achieved by companies that are doing something quite similar today to what they were doing five or ten years ago.
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.