"If the 'ability to make decisions' is a valid definition, then Mitchell simply stating that we make choices routinely is assuming that which he is trying to prove."
I think it's worse than that - I think his take produces an impossible infinite regress.
He paraphrases Sam Harris, who claims that "...we don't know where our thoughts or intentions come from - they just bubble up into consciousness. We have no power over them and can't choose them. Mitchell crucially then claims that Harris is "plain wrong", and that he thinks "...we choose our goals and intentions all the time".
Okay, so let's look at a moment where we choose our goals and intentions. At 13:00 today, I consciously made it a goal that I would get through my work emails, read every one by the end of the day. I consciously made that choice, right? Okay, so let's look at that - why did I consciously make that choice? Well, I had a series of thoughts, maybe something like "My emails have been unread all day, I don't want to leave it until tomorrow when they'll pile up even more, there might be something important in there, so I would be better that I deal with it today than leaving it to pile up." And then I make the decision to do it, right?
So the problem that arises is, even if it's true that I chose to prioritize the goal of reading my emails, it would seem as though I DIDN'T choose to have the thoughts that produced that choice to read my emails.
But, suppose someone says, 'nope, I chose to have those thoughts too'. Okay, so chose them based on what? Chose them because of what preceding chain of thoughts that produced that choice? And did you also choose those thoughts?
If choices are the consequence of thought sequences, then to get around what Sam Harris describes as "thoughts bubbling up", one MUST have an infinite sequence of choices. Otherwise, it must be true that the choice to read my emails was the consequence of thoughts that just bubbled up.
It seems rationally and experientially / intuitively more likely that Sam Harris has a substantial point here, and that thoughts really do bubble up, they really do sort of happen to us.