I was walking down my school hall today and was thinking about just some random things, such as how this hallway smells, who that person looks like, etc. Then, about 2 minutes later I began to think the same basic thoughts, just in a seperate location and at a later time. Since nobody else heard these thoughts the first time, maybe my mind did not really think of them 2 minutes ago but was just telling myself that 2 minutes ago I thought those things. What I mean to say is, how can I be sure that I thought of something earlier if my mind may have just fabricated its own memories?

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You're right: the fact that you seem to remember something doesn't mean it really happened, even if what you seem to remember is your own past thought. And it is not as if you can go back to check. But you can still at least sometimes evaluate the reliability of a particular memory. How plausible is it that the sort of thing I seem to be remembering would happen? How does it fit with other things I believe (even if almost all of those other beliefs are also based on memory)? These are the sorts of checks we run regularly, to decide when to trust our memory and when not to trust it. But it is difficult to see how to block the extraordinary doubt that pretty much all our memories might be wrong. As Betrand Russell pointed out, there seems to be nothing impossible about the idea that we and the rest of the world only came into existence five minutes ago, with our minds pre-stocked with a full set of false memories. That's skepticism for you.

In my salad days, I would have replied: keep an accurate, comprehensive diary; take a lot of photographs; and hang on to all your receipts. Now I know better. None of this solves the logical problem. For example, when you are writing down your thoughts or acts in your diary, are you remembering correctly and hence describing correctly what you thought or what happened? When you later look at the photographs you took during your trip to Mali, how can you be sure [memory?] that these were the images you took there? There is nothing in the image itself that reliably testifies to its own veracity. [No memory-image verifies itself.] The label on your jeans says "Levi" [i.e., not a fake made in Bogalusa], but is there a label attached to the label that says "That other label is genuine, and tells the truth"? Even if there were such a label.... [No label verifies itself.] When God says to you, "I AM THE LORD YOUR GOD," go ahead, be brave, and reply, "Oh yeah? Sez who?" "ASK MY WIFE, YOU DIMWIT." (Jean-Paul Sartre & Bill Cosby.) As I said, the logical problem remains. But the practical problem is solved by diaries, images, and, especially, receipts. You can keep a list of all your wives and lovers (make sure you define "lover" fairly); you can take photographs of all the places you visit, and of all your belongings, and even, page-by-page, of your diary of wives and lovers; and you can buy brown/green accordion-style multifiles and store your receipts alphabetically or chronologically. But make copies of everything, i.e., back up your life. You might not fear the reaper, but watch out for the IRS. Let us return to the logical problem: backing up your life, i.e., making several copies of everything and storing these copies in different places (in case one set gets destroyed, say, in a fire or earthquake) does nothing by way of increasing the veracity of the original. As Ludwig Wittgenstein said somewhere (unless my memory fails me), if the newspaper says that UCLA beat Florida, you don't buy another copy of the newspaper to make sure that UCLA beat Florida. You go online. (Even so, of course....) For more on memory and its problems, see question 150 at http://www.amherst.edu/askphilosophers/question/150.

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