Origins: Developed around the sixth and fifth centuries BCE, in ancient India's ascetic and śramaṇa movements. The Yoga Sutras of Patanjali were compiled prior to 400 CE and presented knowledge about yoga from older traditions into a single system.

Oddity: The Yoga school should really be called an anti-school in that their “doctrine” is that doctrine should flow from empirical investigations, and systematic investigation can only be carried out by doing the following practices. It’s as if a historian of physics, after categorizing the various debating schools of how to interpret the Measurement Problem (the collapse of the state vector into human-observable reality), decided to also include the experimental physicists and lab technicians as its own separate school. Some might even protest,

These lab manuals you’re Xeroxing—they don’t contain any “theory.” These are just how we test theories and interrogate nature. We are not a unified school promoting any specific position.

… and so on. But the historian calls them a school and that’s that.