I agree with Grace pretty much. However, I want to add some more about my understanding in studying Marxism and Capitalism. Yes, human beings are selfish, and selfishness is also the negative force against Marxist non-private ownership because, as Grace mentioned, people only want to work for themselves. As long as they are not rewarded as much as they presumed, they start to be lazy on duty. Therefore, they economical condition will get worse. Furthermore, I want to point out a question in the "a good system of welfare." What should labors deserve? How much they should be rewarded, and based on what, and then they will be rewarded fairly? I want to argue that any "good welfare system" is only a tactful strategy for capitalists continue exploiting labors because they seduce labors to devote themselves to the enterprises by "sweet candies"--welfare--in Lily's example, that will be stocks.
If my understanding is correct, I think Marx suggests that all of profits should be shared and dispatched evenly, so there is no private ownership for every one including capitalists. Private ownership will lead to capitalism because, I think, when every one devotes the same labor into the production line, all of profits should be shared evenly; however, based on what kind of principle, so people can evaluate how much they devote to so that they can shared evenly? As for evaluation, there are various criterion, including 1.working hours? 2.how many products labors made? It comes back to the question about quantity and quality.
There is no right way to do the evaluation. Therefore, how capitalists dispatch the stocks to their workers? In my opinion, I only take the welfare as an improved capitalism to cheat workers to work harder and harder with the ideology that "what you are doing is simply for gaining more bonus or incentives for your own." Through this ideology, labors are twice- or second- exploited because "more work; more gains" succeeds.
Because the basic problem would be the total bonus is divided to be shared with everyone, including investors, owners, and labors, etc. But, as you can easily realize, labors share the least portion of all bonus. So my next concern is: Should not labors share all of bonus evenly with investors and owners? If they could not, with what reason that they could not? The truth is, according to Capitalism: No, they never could. Therefore, labors of course will turn to ask for private ownership. So, the problem of private ownership comes back.
I want to stop this dialectical problem because I cannot solve this problem at all. I think this problem can never be solved. Many many things have changed from his time. For instance, in Marxist time, there certainly was no high-tech. Marx would never know that high-tech engineers must spend much time and money in studying for getting a degree, he neither can know that engineers only produce programs, which is the result of spending long long time but without any "laboring jobs." How Marx would evaluate "products of wisdom"? I wonder how he would revise his theory.