There are a few parts to this, and I realize I could have been more clear. The first is that they are invisible "to you" through introspection. To explain this, first let's show that nearly all your mental events happen unexpectedly (to you). You generally don't know what thought or feeling you are about to have next. You may try to set it up by saying "I will think of an elephant", but then your next thought isn't just "an elephant", but rather "see? I thought of an elephant!", or some other mixture of thoughts which is not what you expected. Your act of predicting ruined the prediction. And this is for cases where you try to force it. When you don't even try, it's completely unexpected. There may be incredibly rare exceptions where you get lucky.

So if you accept that (nearly) all your mental events come unexpectedly, then you are always reacting to the event after the fact, and struggling to figure out what happened retroactively. It's like when you aren't paying attention during a movie, miss something, and then try to figure out from the current stuff on the screen what you missed. (e.g. he's celebrating and running down the pitch, did he score a goal? Did his friend score it?...) Except this is happening continuously, and you are always behind.

This is true even if you have a theory for what mental events are. Your interpretation of the event (e.g. "this is a feeling brought about by a thought of home") will necessarily happen after the event itself. this means you could be wrong, but can't go back in time to check it, and it will never happen exactly the same way again.

Hope that helps