Is this a good time to invest in the share market?

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The world economy being the way it is today, and with nasdaq, sensex and other bourses being on a decline, is this a good time to invest in shares and stocks, or should one wait some more? My close friend plays the stocks actively and is considering investing heavily during this phase. I am not so sure and feel that he should wait for while and watch. What are your thoughts, opinions and advice on this?