Here's the thing.....and keep this rule as a constant reminder....You will always do the exact same thing under the exact same circumstances. Not similar EXACT. You could argue that there really is no way to test that theory. Because it never will be the EXACT same circumstances. But you can be "close" and with all the other information, logic dictates that my theory (at least) holds some water.
So lets begin with the simple stuff...What did you eat today? whatever the answer, follow it up with this: Why did you eat that? and finally, why would you have eaten anything different? Now here is the trick: you are not allowed to use the word "if" when you answer that last question (or any words which would change the situation). and really the answer is no you would not have eaten anything but what you did eat because of the reasons you gave in your second answer. To go one step further one can argue that at the same point in time you would have always ate what you ate because there is no reason why you wouldn't. Think about it.
lets take bob here (some guy i made up) he's at his job and he's hungry. Now it's 10AM and his lunch break is at 11AM. he could go on an early lunch and eat. and this decision to go early is already decided before he makes it. See everything that led up to that decision will make him choose a certain path. Watch this: "IF" he had taken an early lunch previously and his boss told him to never do it again or he's fired, then he's not gonna do it, unless he wants to get fired. "IF" his is at a job where he can take an early lunch then he might do just that. or "IF" the lunch place doesn't open till 11AM. I could go on but as you can see there are tons of "IFs" one can think of to either have him go to lunch early or not.
Bob doesn't go to lunch early. For whatever reason he doesn't and on that day at that time he will never go to lunch early. why would he? and remember, you can't use the word "IF"
I hope that makes some sort of sense (I tend to find that my theories sound better when I explain them person to person) My mind goes at least 100 times faster than my fingers as i type!)