I would say he only puts things in our mind via what we think about the physical world. So then only the thoughts that are portrayed in the physical world are available to Satan. Not the ones we don't display. Remember though we can display them in seamingly invisible ways, but Satan is sharp.
But one might then think, that if our thoughts are chemical and energy based, that they are physical, and so Satan could read them.
Is this question answered in scripture?
Just a little rant as I think:
~But then it seams that evil does not originate form Satan, but rather, I'd blame the result of how the bodies that God created react with the world, and come to think evil things. I don't want to say this, but then wouldn't that mean evil is God's mistake? God created Adam and Eve and the earth in such a way that they could choose between good and evil. So then why didn't God just create us without the option of evil to simply reign with Him in the first place?
Well, we can do that for eternity. We only have 90-ish years here, so why not enjoy it. It'll be something different form the eternity afterwards. lol. And it lets us be in heaven by choice. Lets us be dead before we are alive so we can appreciate being alive.~
But we'll see how His plan was all perfect once we get there. For now I trust Him. Not my wondering mind.