The second justification for IP is the stimulation of creation theory. It starts with the idea that authors and inventors are motivated by (the possibility of) financial gain. Then it reasons that under unregulated free market conditions people will copy and share without paying writers or inventors. To fix this, authors and inventors are given a monopoly over their creations, so they can extract money from the market. And by making creation thus (more) profitable, more people will be inclined to create. In philosophical terms this approach is utilitarian, as it is primarily about consequences, not about rights.
Utilitarianism as introduced by William Bentham was a real improvement over previously held ethical theories, as by looking at the consequences things had for the general well-being, instead of for God, or society as it existed, it was the first ethical theory that put normal human beings at the center. There may be theoretical problems with utilitarianism such as how welfare is to be defined, and to what degree an increase in welfare for one person or many persons, can legitimately weight up to that of another, but if kept in check by common sense, it is quite useful. And it can deliver a quite sensible justification for IP.
But the stimulation justification nevertheless hinges on two assumptions that can be questioned. The first is whether most or all authors are indeed motivated by direct financial gains. Even if some are, there might still be enough intrinsically, or differently motivated authors left to provide for new cultural works (as we see in the competition between bloggers and news-paper journalists). The second is that scarcity (a monopoly property right) is the only feasible way to extract money from the market. While historically, in the age of printing, this may have been true, we will argue that while the first assumption at least partially does hold, the second is not an a-priori given, and certainly does not need to hold any more.
The influence of the stimulation of creation justification can be seen in the fact that practically everywhere copyrights are for a limited time only, to eventually benefit the cultural commons. Also the use of monopolies to stimulate creation is explicitly mentioned in the American constitution: "... securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries ... to promote the Progress of Science and useful Arts". The interests central to the stimulation justification are those of the public and society.