1. By observation and attention.
Not only the ancients, even you and me have learnt ONLY this way. Rest all is knowledge, coming from outside, with a different use. We learn by observation and attention only. The reminders all are present in the external world. The sky, the streams, the changing seasons, the acts of the mind, the bodily senses, birth and death, the entire movement of nature. Anybody who just pays attention sees the Truth.