I came across a problem:

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> Consider a painter to be standing on a movable platform that is attached to rope that goes over an overhead pulley. The free end of the rope is in the man's hand. He pulls the rope with a force F so that the system is accelerated upward.
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My confusion arises from the statement that "internal forces can't accelerate the system". In this case, however, the force applied on the rope by the man is clearly an internal force. Notwithstanding, the system is accelerated upward. How does this happen? 

Please help! Thanks in advance :) Regards.