It is interesting to apply probability theory in various situations in which we can’t always reduce it to numbers very well, but still it shows automatically what kind of information would be relevant to help us do plausible reasoning. On the basis of no knowledge but this, e(Guilty|X) = −70 db is the plausibility that any particular person is the guilty one. How much positive evidence for guilt is necessary before we decide that some man should be put away? Perhaps +40 db, although your reaction may be that this is not safe enough, and the number ought to be higher. Probability theory sayse(Guilty|Motive)=e(Guilty|X)+10log10P(Motive|Guilty)P(Motive|Not Guilty) (5-38)≃−70−10log10P(Motive|Not Guilty)since P(Motive|Guilty)≃1, i.e.