Without a core philosophy you're not going to be able to hold on to your positions or stick with your trading plan during really difficult times. You must fully understand, strongly believe in, and be totally committed to your trading philosophy. In order to achieve that mental state, you have to do a great deal of independent research. A trading philosophy is something that cannot just be transferred from one person to another; it's something that you have to acquire yourself through time and effort.
"A lot of great fortunes in the world have been made by owning a single wonderful business. If you understand the business, you don't need to own very many of them."
Buffett means outright ownership of a buisness not being a just a stock holder.But owning the right stocks if you don't want to own/run a buisness is very important. Peter Lynch once said that he never met a stock he didn't like he owned more stocks when he ran his fund than any of his peer's at the time and his record was outstanding. So you can own a few or you can own alot you can be a Bull or Bear;Trader or Investor;Follow T/A or Fundamentals ; own or rent and still show a profit.