Every use of the work of other

First of all, 'every use of the work of others is theft' is not true, though certainly there are illegitimate uses of others' works.

Legitimate:
1 For non-commercial use, you can ask the copyright owner to grant you permission to use his work.
2. For commercial use, buy the copyright from the copyright owner or set up a contract and share the benefit gained by using the owner's work.
3. There are certain laws allowing uses of someone else's work for specific purposes. For example, I've been in a country where teachers and students can photocopy a single chapter of a novel or quote less than 5% of an academic work; police can, through legal procedures, request the use of someone's work, etc.
4. For literary and scholarly works, there are rules to avoid plagiarism.
5. Once the copyright expires, that previously copyrighted idea/thing can be used by others. For example, some copyrighted industrial inventions expire in 5 years. After that no one can re-possess the copyright, so that the other companies can benefit from the invention and thereby increase efficiency.

Illegitimate:
1. Re-produce copyrighted material without permission from the copyright owner.
2. Sometimes 'reverse-engineering' is against the law. Reverse-engineering involves analyzing the original invention to gain knowledge and techniques so that a similar product can be re-invented.
3. Imitating copyrighted logos and slogans are illegitimate. There is a design company in HK named Lego Design (something like that), and soon it was sued by Lego from Europe and had to pay a fine.
4. Schools and universities have rules against plagiarism. Plagiarism is citing, using, or paraphrasing another scholarly work or copyrighted material without recognizing the original author/editor/inventor.
5. There are times the original inventions were stolen/copied by someone else before they were copyrighted. There are copyright disputes to sort out who should possess the copyright.

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