Back to philosophy. I see no reason to suspect an outside source. Who would want to control your mind, anyway? Most thoughts are in response to stimuli.
Your sensory organs constantly supply the brain with data that has to be processed. Your brain maintains a database of experiences. One has to consult that
database when analyzing current sensory data. It is all very thought provoking. Before you have much of a database, ie., when you are a toddler, you probably
don't have a whole lot of analysis going on in there. I have noticed, that when the brain is ill, the thought creation process is jaundiced. Impulsive thoughts and
intrusive thoughts are not uncommon. Kind of like the burps from an upset stomach.