I concur in your assessment; this is a premise. A fundamental one, too.
I declare being cought in the notion that your mind is a (pure) function of input to be self-deception, which will ultimately lead to a path of misery.
If this were not true, how come that many people experience a change in themselves when they meditate? If you haven't been exposed to Buddhism yet, for example Zen meditation (Zazen) has the ultimate goal just to sit and breathe. Thoughts and external input are to be dismissed during Zazen.
Telling from personal experience, this practice changes people. Without any external input whatsover. Hence: The human mind cannot be a pure function of external sources.
