"... if the objective reality of any one of my ideas is of such a nature as clearly to make me recognize that it is not in me ..., and that consequently I cannot myself be the cause of it, it follows ... that I am not alone in the world, but that there is another which exists, or which is the cause of this idea." (IP3 52a)

So Descartes' strategy to get out of the solipsistic predicament and prove that he is not alone in the universe is to find an idea which must be caused by something other than himself. To prove it the idea must be caused by something other than himself he will appeal to the causal principle for objective reality.