The Rights of Sentience was a clause of the Galactic Constitution. It spoke out against speciesism and set out the rights that all members of the Galactic Republic were entitled to, including these:
When the Constitution was written in 25,053 BBY, the Rights of Sentience was one of its most radical sections. However, throughout its history, the Republic was hard-pressed to enforce the Rights, especially in its outlying regions. In the New Order, it was replaced with Human High Culture but still legally in effect. The Rights of Sentience were still practiced by the Rebel Alliance in defiance of the Galactic Empire, who reinstated it when the New Republic Common Charter was drafted. Senate Resolution 54.325 most likely helped enforce the law. After Admiral Daala weakened Human High Culture, it can be presumed that the Rights found a place in the Imperial Charter.
It is not known how the Galactic Republic determined which species were sentient. Such proved a problem for the droid rights movement, as droid sentience was questionable at best.