system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it legal for me to share a textbook with a friend but illegal for me to simply pirate a copy?

In either case, I'm using a textbook I haven't paid for and my friend has still paid for it. Is it convenience that's making it illegal? If so, what if my friend was my roommate? That would take the convenience factor out of the equation since I would have easy daily access to the textbook.

A related (possibly same) question. Why is it legal to share a textbook with a couple of friends and/or strangers in person and illegal for me to share a textbook with friends and/or strangers over the internet? Aren't they essentially the same thing other than convenience? Depending on the situation, either case could be more convenient than the other.
gpt
Because the First Sale Doctrine says that the publisher of a book loses physical control of the book after he sells it, after that, the purchaser is completely free to dispose of the book as he wishes (including loaning it out or destroying it).

But the law explicitly forbids copying the book without permission.