The development of new inventions is promoted by the granting of patent rights, which restrict the right of anyone but the patent holders to profit from these inventions for a specified period.
Conclusion
If we stopped granting patent rights, no one would engage in original development and no new inventions would happen.
Intermediate Conclusion
If we stopped granting patent rights, inventors wouldn't have any financial incentive to invent.
Evidence
If we stopped granting patent rights, anyone could just copy someone else's invention.
Evaluation
The author seems to be assuming that if other people can copy your invention, then that will greatly reduce or eliminate your financial incentive for inventing. So he must be assuming that, and that
In addition, even if we grant that other people would saturate the market with copies of your invention and you'd barely be able to make any money yourself, we still have to assume that people won't invent without a financial incentive.
But don't a lot of inventors just like the problem-solving process? Or maybe they're inventing for the fame that comes from inventing something useful. Maybe they're inventing for the sake of saving someone's life?
Goal
Look for an answer relating to the intermediate conclusion (assuming that people who copy would also try to sell your invention and that doing so would significantly cut into your own ability to achieve financial gain from your invention), or one relating to the main conclusion (that if you don't have a financial incentive, you won't bother inventing / people don't invent for non-financial ends)