1 Default Reasoning Q: How many wheels does John’s car have? A: Four (by default) The conclusion is withdrawn if one is supplied with the information that one wheel of John’s car has just been stolen. non-monotonicity
2 Default Reasoning Q: Can bird Tweety fly? A: Yes (by default) The conclusion is withdrawn if it is added that Tweety is a penguin. non-monotonicity
3 Default Reasoning Q: How many wheels does John’s car have? A: Four (by default) The conclusion is withdrawn if one is supplied with the information that one wheel of John’s car has just been stolen. car(x) … wheel-num(x, 4)
4 Default Reasoning Q: Can bird Tweety fly? A: Yes (by default) The conclusion is withdrawn if it is added that Tweety is a penguin. bird(x) … fly(x)
5 Default Reasoning Q: How many wheels does John’s car have? A: Four (by default - CWA) The conclusion is withdrawn if one is supplied with the information that one wheel of John’s car has just been stolen. car(x) abnormal(x) wheel-num(x, 4) wheel-stolen(x) abnormal(x)
6 Default Reasoning Q: Can bird Tweety fly? A: Yes (by default - CWA) The conclusion is withdrawn if it is added that Tweety is a penguin. bird(x) abnormal(x) fly(x) penguin(x) abnormal(x)
7 Default Reasoning Usually, Quakers are pacifist Usually, Republicans are not pacifist Richard Nixon is both a Quaker and a Republican Q: Is Nixon a pacifist?
8 Default Reasoning Usually, Quakers are pacifist Usually, Republicans are not pacifist Richard Nixon is both a Quaker and a Republican Nixon pacifist Quaker Republican Nixon diamond
9 Default Reasoning Usually, Quakers are pacifist Usually, Republicans are not pacifist Richard Nixon is both a Quaker and a Republican quaker(x) abnormal 1 (x) pacifist(x) republican(x) abnormal 2 (x) pacifist(x) quaker(Nixon) republican(Nixon)
10 Default Reasoning Default consequence: KB D iff is true in every preferred model of KB.
11 Default Reasoning quaker(x) abnormal 1 (x) pacifist(x) republican(x) abnormal 2 (x) pacifist(x) quaker(Nixon) republican(Nixon) quaker(Nixon) republican(Nixon) abnormal 1 (Nixon) abnormal 2 (Nixon) … quaker(Nixon) republican(Nixon) abnormal 1 (Nixon) abnormal 2 (Nixon) … more preferred to Model 1 Model 2
12 Default Reasoning Uncertain reasoning: car(x) wheel-num(x, 4) [l 1, u 2 ] bird(x) fly(x) [l 3, u 4 ] quaker(x) pacifist(x) [l 5, u 6 ] republican(x) pacifist(x) [l 7, u 8 ] [l, u]: probabilistic or possibilistic intervals