Stuart Russell

But on the other hand, suppose you had a choice between two moves.

One of them you've already figured out is guaranteed to be a draw, let's say.

And then the other one looks a little bit worse.

It looks fairly likely that if you make that move, you're going to lose.

But there's still some uncertainty about the value of that move.

There's still some possibility that it will turn out to be a win.

Then it's worth thinking about that.

So even though it's less promising on average than the other move, which is guaranteed to be a draw,

There's still some purpose in thinking about it because there's a chance that you will change your mind and discover that in fact it's a better move.

So it's a combination of how good the move appears to be and how much uncertainty there is about its value.

The more uncertainty, the more it's worth thinking about because there's a higher upside if you want to think of it that way.

Yeah, so the early parts of the search tree are fairly bushy, that it would look at a lot of different possibilities.

But fairly quickly, the degree of certainty about some of the moves, I mean, if a move is really terrible, you'll pretty quickly find out, right?

You'll lose half your pieces or half your territory.

And then you'll say, okay, this is not worth thinking about anymore.

So further down, the tree becomes very long and narrow, and you're following various lines of play, 10, 20, 30, 40, 50 moves into the future.

And that, again, is something that human beings have a very hard time doing.

mainly because they just lack the short-term memory.

You just can't remember a sequence of moves that's 50 moves long.

And you can't imagine the board correctly for that many moves into the future.