Depends what you mean....
Longer term buy & hold - tis feasible for an amature investor to pick say 10 stocks and outperform the FTSE 100 over say 5-10 years.
As for short term trading/speculation- tis a lot harder and a lot less forgiving, costs & execution can become a lot more of a factor and these days, with the prevelance of spreadbetting, leverage also becomes something to be aware of - tis not very feasible for an amature to make money - most people who try will lose.