Gettier cases

We have said that knowledge is justified true belief. However, as you're no doubt aware by now, philosophers like to get to the bottom of things, and this can often mean throwing the spanner in the works of an apparently satisfactory theory. We are going to end this chapter by discussing some cases that cast some doubt on the plausibility of the intuitive theory that knowledge is justified true belief.

These sorts of cases, in which we have a true belief for which we have adequate rational justification yet can't really say that we have knowledge, could occur in a court case:

Imagine that a jury in a murder case, having carefully considered the evidence, form the belief that the defendant is guilty. The strongest piece of evidence they have for their belief is the fact that the defendant's blood was found at the scene of the crime and on the gloves also found there. As it turns out, the defendant did indeed commit the murders of which they stood accused.

Thus, the jury have a belief, it's true and they have rational justification (in the form of the blood, gloves and other evidence) for it. However, suppose that the blood at the scene was planted by the investigating police officers in an (albeit unnecessary) attempt to incriminate the defendant.

Now, if the jury convict on the basis of the forensic evidence concerning the blood found at the crime scene, would you be comfortable with the conviction - would you feel that the jury are really justified in convicting?

Cases such as these should encourage us to reflect further upon the adequacy of the tripartite account of knowledge. Indeed, some philosophers are fascinated by such cases. In a famous essay Edmund Gettier devised a set of cases that lead us to question whether rational justification of a true belief is sufficient for knowledge.9 The cases are quite complex and rely upon the rules governing logical relations between certain sorts of propositions. The following Gettier case relies upon the rules of logical entailment for propositions that are joined by the connective word 'and'. It is fairly complex, but try to follow and see why the satisfaction of the three conditions for knowledge given by the tripartite account doesn't seem to be sufficient to say that there is knowledge in this case.

Suppose that Sherlock the detective, investigating a murder case, has the following belief:

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