To quote from the logician/philosopher Bertrand Russell:.
"All traditional logic habitually assumes that precise symbols are being employed. It is therefore not applicable to this terrestrial life but only to an imagined celestial existence-.
If we are to create plans that work reliably in real world environments, which is one of the main goals of the planning community, then we need to move away from Classical Planning' and deal with the problems brought about by uncertainty. .
What do we mean by uncertainty?.
When most people talk about uncertainty, they are referring to a condition of doubt. To quote from The American Heritage ® Dictionary of the English Language':.
Uncertainty:.
1) The condition of being uncertain; doubt.
2) Something uncertain: the uncertainties of modern life. .
3) Statistics. The estimated amount or percentage by which an observed or calculated value may differ from the true value.
In the case of planning under uncertainty, all of these definitions are relevant, we certainly wish to deal with the uncertainties of modern life, and as we shall see later, the statistical chances of an event happening are of particular relevance when using certain modelling techniques.
To be completely clear as to what we might mean when we talk about uncertainty, let us look at a simple example.
Suppose we have a set of four cards: the ace and two of hearts, and the ace and two of spades. After a fair shuffle of the deck, two cards are dealt to Alice. It is easy to see that, at this point, there is a probability of 1/6 that Alice has both aces, probability 5/6 that Alice has at least one ace, a probability ½ that Alice has the ace of spades, and probability ½ that Alice has the ace of hearts: Out of the six possible deals of two cards out of four, Alice has both aces in one of them, at least one ace in five of them, the ace of hearts in three of them, and the ace of spades in three of them.
