Can thought be considered a process that begins after being affected by an external stimulus? And without prior experience on which to base our response, we can only react according to the conditions set by that stimulus?
thought must be influenced by some level of internal stimulation, as you can deprive your senses and still think. Likewise, before you have the capability of memory, you are not conscious, it is simply impossible. Consciousness is directly tied to the ability to remember. Thusly it must be some mix of the two, life experiences, and remembered experiences.
Just one or the other is incapable of providing anything interesting.
You have to have some level over the control of your thoughts, otherwise you wouldn’t be capable of typing, or speaking, or most things in life really. If everything is predetermined it has to be predetermined at a higher level, quantum mechanics and multiverse theory for example. In some capacity, the vast majority of judgements we make in our lifetimes is based on the cumulative life experience we have gained, as well as a ability to engage in critical thinking allowing us to process said experience and to formulate an action to follow.
We who are so easily influenced into outrage by trigger phrases specific to our fears or spurred into action by resonating soundbites promising our desires, are those our thoughts or are they just the mind left to its own devices?
both, kind of. The mind left to it’s own devices will formulate thoughts for you to hold, it doesn’t like the lack of thought and understanding, it’s probably the reasoning behind religion, and it’s certainly the driver behind science. We’re also social animals, and as a result we like to conform to the pack, it’s what keeps us alive, so if other people are saying something, we have some fundamental level of bias against the alternative. Racism is a good example of this.