When it is fairly certain that the patent is invalid, a patent can be challenged in a different way, for example, by going ahead and manufacturing or using the product anyway, and waiting for the patent holder to sue. As noted above, patent rights are private rights. Patent holders have to take action if they wish to protect their rights. If they do, the invalidity of the patent is then raised as a defence. If they don't, for whatever reason, then the matter may go no further