I like what Bertrand Russell had to say to that very question, in one of Unpopular essays (quoting approximately from memory):

Philosophy is science in the circumstances of insufficient knowledge. That is, given insufficient data it may still be beneficial to ponder possible future directions of research, contemplate lines of attack on long-standing questions, or even determine beforehand how these big questions should be posed. All of these circumscientific activities and thinking together constitute philosophy.