Mr. True-Temp

An intriguing philosophical puzzle from Keith Lehrer:

Suppose a person, whom we shall name Mr. Truetemp, undergoes brain surgery by an experimental surgeon who invents a small device which is both a very accurate thermometer and a computational device capable of generating thoughts. The device, call it a tempucomp, is implanted in Truetemp’s head so that the very tip of the device, no larger than the head of a pin, sits unnoticed on his scalp and acts as a sensor to transmit information about the temperature to the computational system of his brain. This device, in turn, sends a message to his brain causing him to think of the temperature recorded by the external sensor. Assume that the tempucomp is very reliable, and so his thoughts are correct temperature thoughts. All told, this is a reliable belief-forming process. Now imagine, finally, that he has no idea that the tempucomp has been inserted in his brain, is only slightly puzzled about why he thinks so obsessively about the temperature, but never checks a thermometer to determine whether these thoughts about the temperature are correct. He accepts them unreflectively, another effect of the tempucomp. Thus, he thinks and accepts that the temperature is 104 degrees. It is. Does he know that it is?

(Keith Lehrer, Theory of Knowledge, (Westview Press, 1990): pp. 163-164.)

(Here's a grossly over-simplified explanation.)
The puzzle is intended to question a certain understanding of knowledge called externalism. Knowledge is usually taken to be justified-true-belief, but the question is, what makes a belief justified? Internalists will say that you need reasons to be justified (these are internal to your mind). Externalists will say that you are justified if your beliefs were formed in the right way (i.e., a cause and effect story, which is external to your mind). Now, if your belief has been formed in the correct way, especially a reliable way, then the externalist will say that you have knowledge. But Mr. Truetemp, whose beliefs about the temperature are formed in a perfectly reliable way, doesn't seem to have knowledge, because he has no idea why he believes these things. This is a difficulty for the externalist.

Makes you want to be a philosophy major, doesn't it?