Non-initiation of force
No one may start the use of physical force against others.
Rand held that force is the opposite of reason, and that people must deal with one another by persuasion and consent. Force is legitimate only in retaliation, against those who start its use, and should be placed under objective control by government.
Read it in Rand’s words
Atlas Shrugged (Galt's speech); The Virtue of Selfishness