THE Forest philosophers
Let's first get an overview of the era:
From the 7th century BC, a great number of Indian philosophers departed to the forest, where they lived a life of solitude and austerity, leaving behind their family, wealth, and status.
In the forest, they dedicated themselves to studying the nature of reality by engaging in intellectual discussions and meditation.
Their teachings were written down in a series of texts called the Upanishads—meaning “to sit down beside”, referring to the passing of wisdom from teacher to student.
Earlier texts had identified the soul (atman) with the breath or with the senses, but the forest philosophers no longer deemed these definitions satisfactory.
Instead, they identified the soul (atman) with the unconscious—the space inside of ourselves from which our thoughts and feelings arise.
They rightly concluded that this reality could not be observed directly, as we “cannot see the seer who does the seeing.”
Meet sage uddalaka
Now let's see this period through the eyes of an individual:
In the 8th century BC, the sage Uddalaka asked his son Svetaketu to open a seed from a tree, only to discovered nothing inside. Just as all our thoughts and feelings arise from the nothingness of our unconscious, so to the entire tree appeared from the nothingness inside the seed. He spoke:
"Bring me a seed from the nyagrodha tree, break it and tell me what you see."
"Nothing at all, sir."
"My son, yet the entire tree formed from that subtle essence which you do not perceive there. In that same essence all that exists has its self. That is the Atman. And you Svetaketu, are That."