What do you think about this ogy to why Keynesian economics are wrong?

Say a fisherman wants to increase the amount of fish he catches each day. To do so he needs to build a larger net. But in order to build the larger net he has to take time off from fishing. So in order to proceed with the building of the net, he has to *save* more fish than what he catches on average, so he has a pool of resources to draw from when away from his normal day of fishing. He would have had to have saved them beforehand. He can't create fish out of thin air as he is away from fishing and building the new net.