We begin with an example from Panayot Butchvarov's The Concept of Knowledge, Northwestern University Press, 1970, p. 47. [CK is the red volume on the topmost visible shelf. Immediately to its right is Butch's Being Qua Being. Is Butch showing without saying that epistemology is prior to metaphysics?] There is a bag containing 99 white marbles and one black marble. I put my hand in the bag and without looking select a marble. Of course, I believe sight unseen that the marble I have selected is white. Suppose it is. Then I have a justified true belief that a white marble has been selected. My belief is justified because of the fact that only one of the 100 marbles is black. My belief is true because I happened to pick a white marble. But surely I don't know that I have selected a white marble. The justification, though very good, is not good enough for knowledge. I have justified true belief but not knowledge.
Knowledge, says Butchvarov, entails the impossibility of mistake. This seems right. The mere fact that people will use the word 'know' in a case like the one described cuts no ice. Ordinary usage proves nothing. People say the damndest things. They are exaggerating, as a subsequent post may show. 'Know' can be used in non-epistemic ways — think of carnal knowledge for example — but used epistemically it can be used correctly in only one way: to mean absolute impossibility of mistake. Or as least that is Butchvarov's view, a view I find attractive.
Admittedly, knowledge as impossibility of mistake is a very stringent concept of knowledge. Why should we care to set the bar so high? Why is knowledge in the strict sense important? It is important because there are life and death situations in which one needs to know in order to decide on a course of action.
Suppose I am calmly and rationally contemplating suicide. I have lived a full life, my spouse has died, I have no dependents, and I have accomplished all that I can accomplish. But I am old and worn out and I have been diagnosed with a painful form of terminal cancer. Should I do away with myself? The money saved on expensive treatments could be donated to a worthy cause, etc.
If I know that I am just a material being with no prospects of post-mortem survival, then I have nothing to fear from God or an afterlife, and no need to factor in the moral progress I could make by bravely enduring the terminal suffering. For if I am nothing but a complex natural organism, then there is nothing to hope for beyond this life, and that moral progress could do me no good. (The little good it would do others is a vanishing quantity in comparsion to the suffering I would have to endure.) Now is it not obvious that knowing whether one will or will not survive is very important? Clearly, what one wants here is genuine knowledge, which implies impossibility of mistake. For one is about to make a very important decision, perhaps the most important decision one can make.
Kant asked four great questions: What can I know? What ought I do? What can I hope for? What is man? One makes a joke of the first question if one demands anything less than certain knowledge. And note that what one wants is knowledge with respect to the other questions, not mere conjecture. Butchvarov puts the point as follows:
Where the truths in question are of the greatest importance, as philosophical truths usually are, where what is at issue is the immortality of the soul and the possibility of eternal damnation, the existence of an external world, of other persons, God, or of a real past, mere evidence, however good, is not enough — it is knowledge, impossibility of error, that we demand. (CK, 271)