"Intellectual property" (IP, not to be confused with IP address) is a toxic capitalist idea that says that people should be able to own information (such as ideas, presentation style, songs or text) and that it should be treated in ways very similar to physical property. For example patents are one type of intellectual property which allow an inventor of some idea to own that idea and be able to limit its use and charge money to people using that idea, or prevent people from using that idea altogether. Copyright is probably the most harmful of IP today, and along with patents the most relevant one in the area of technology. However, IP encompasses many other subtypes of this kind of "property" such as trademarks, trade dress, plant varieties etc. IP is an arbitrarily invented grant of monopoly on information, i.e. something that is otherwise naturally free.
Most people with brain oppose this idea, see e.g. http://harmful.cat-v.org/economics/intellectual_property/.
IP exists to benefit corporations, it artificially limits the natural freedom of information (see artificial scarcity) and tries to eliminate freedom and competition, it fuels consumerism (for example a company can force deletion of old version of its program in order to force users to buy the new version), it helps keep malicious features in programs (by forbidding any study and modifications) and forces reinventing wheels which is extremely energy and resource wasting. Without IP, everyone would be able to study, share, improve and remix and combine existing technology and art.
Many people protest against the idea of IP -- either wanting to abandon the idea completely, as we do, or at least arguing for great relaxation the insanely strict and aggressive forms that destroy our society. Movements such as free software and free culture have come into existence in protest of IP laws. Of course, capitalists don't give a shit. It can be expected the IP cancer will be reaching even more extreme forms very soon, for example it will be perpetual and encompassing such things as mere though (thoughts will be monitored and people will be charged for thinking about ideas owned by corporations).
It must be noted that as of 2020 it is not possible to avoid the IP shenanigans. Even though we can eliminate most of the harmful stuff (for now) with licenses and waivers, there are many things that may be impossible to address or posing considerable dangers, e.g. trademark, personal rights or patent troll attacks. In some countries (US) it is illegal to make free programs that try to circumvent DRM. Some countries make it explicitly impossible to e.g. waive copyright. It is impossible to safely check whether your creation violates on someone else's IP. There exists shit such as moral rights that may exist even if copyright doesn't apply.
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