Especially effective are some examples he gives comparing software to other copyrighted works, such as a novel:
A novel and a modern complex programme have certain points in common: each is large and implements many ideas. Suppose patent law had been applied to novels in the 1800s; suppose states such as France had permitted the patenting of literary ideas. How would this have affected Hugo's writing? How would the effects of literary patents compare with the effects of literary copyright?
Consider the novel Les Misérables, written by Hugo. Because he wrote it, the copyright belonged only to him. He did not have to fear that some stranger could sue him for copyright infringement and win. That was impossible, because copyright covers only the details of a work of authorship, and only restricts copying. Hugo had not copied Les Misérables, so he was not in danger.
Patents work differently. They cover ideas - each patent is a monopoly on practising some idea, which is described in the patent itself.
Here's one example of a hypothetical literary patent:
Claim 1: a communication process that represents, in the mind of a reader, the concept of a character who has been in jail for a long time and becomes bitter towards society and humankind.
Claim 2: a communication process according to claim 1, wherein said character subsequently finds moral redemption through the kindness of another.
Claim 3: a communication process according to claims 1 and 2, wherein said character changes his name during the story.
If such a patent had existed in 1862 when Les Misérables was published, the novel would have infringed all three claims - all these things happened to Jean Valjean in the novel. Hugo could have been sued, and would have lost. The novel could have been prohibited - in effect, censored - by the patent holder.