Seeing isn't believing. Not anymore.

I started thinking about this when I saw a video clip from the first moon walk. I have to say, it looked a little cheesy -- about on a par with the special effects of Star Trek (and I mean the original series).

Not long after that, I watched a rerun of "Terminator 2," the science fiction movie blockbuster. I saw a man walk through steel bars, just ooze right through them, and recongeal on the other side. I believed it.

But the moon walk happened. The shape-shifting liquid metal android from the future did not. (I think.)

Part of me longs to believe those great photographs on the covers of tabloids: the "alien talks to Clinton" photos, or the half- alligator child, born to a seven-year old.

But even National Enquirer has admitted -- when it ran the photo of Nancy Kerrigan and Tonya Harding skating side by side, almost hand in hand -- that a cover photo can be faked. These days it doesn't really take all that much technical expertise to splice two photos together. When such products are created with care, even the pros can't swear that the picture is real.

The day has almost arrived when videotapes and photographs cannot be accepted as evidence in court. Connie Willis, a science fiction writer and a friend of mine, maintains that this highlights a new and heretofore unrealized purpose of the library: to maintain the evidence. When an unscrupulous publisher comes out with a book showing that Hitler and F.D.R. had lunch together in Munich just before the invasion of Poland, the only way more ethical researchers can disprove it is to go back and examine the sources. That's assuming that the original photographs can still be found.

The same point can be made about historical documents. I spend a lot of time on the Internet -- my wife would say, too much time. I've watched a sort of electronic credulity come into being: a lot of people seem to believe that if you find something on the World Wide Web, then it must be true.

You can see how this might be dangerous. Suppose in our haste to digitize our documents, we scan in the United States Constitution, then destroy the original paper. If somebody then goes in and changes the Constitution on-line -- removes those pesky Amendments, for instance, or even just changes a phrase or two -- how would you know?

The answer: only by comparing it to text that pre-dates the on- line version. In this age of instant information, just who is going to take the time to do that? The danger is greater with less well-known materials -- local laws and policy statements, for instance. Why? Because they haven't been as widely disseminated. The electronic version may be the only version.

For a long time, I have held the philosophy that the public library must keep its inventory current. A well-used library gets "weeded" regularly; unused materials get pulled and placed into book sales. Mostly, I still believe that. Public libraries are not museums; they are cultural retail outlets.

But lately I'm beginning to look up a few basic historical documents, just to make sure that the library still has a copy. Like I always say, reading is believing.