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Source Code
Source code is like the blueprints for a piece of software. Source is software in a human readable and human modifiable form. The alternative to source code is “machine code.”
Two of the freedoms that I enumerated [(1) the ability to study a program and adapt it and (2) the ability to distribute changes] require access to source.
Restriction of access to source code is done both technically and legally.
The famous car metaphor:
Would you buy a car whose hood it was illegal to open?
Would you by a car with the hood welded shut?
What if breaking the weld points was a criminal offense?
This is similar what you are doing when you buy, support and develop non-free software.