In addition to a true belief that p, Robert Nozick proposes two more conditions for some person S’s having knowledge:
(3) If p were not true, S would not believe that p.
(4) If p were true, then S would believe that p. (Philosophical Explanations, 172ff)
(3) implies that S’s evidence must not lead him to believe that p regardless of whether p is true or false. In the unhappy case in which it does so lead him, since one can’t in principle have good evidence for false beliefs, S’s evidence for p is not, in fact, good, even if the belief happens to be true. Hence S’s belief is unjustified. In other words, the belief is not “sensitive” to truth; the truth of p does not cause S to believe that p, through whatever means or intermediaries.
(4) is relative to a method of knowing. For example, I have a JTB that there is a pen on the desk because I see it. But not in all cases where the pen is on the desk would I know this, because I might not be looking at it or might overlook it, etc. Yet we would still like to say that I know that the pen is on the desk. So, we must fix the method of knowing, e.g., if p were true, then if learned that p via method M (such as seeing the pen), I would infallibly come to believe that p. At the same time Nozick says that for fundamental beliefs like “the world has existed for many years already,” no particular method needs to be specified: “[s]o nested are these statements in our other beliefs and activities, and so do they nest them, that our belief or acceptance of them is… best represented apart from any particular methods.” (185)
Example 1. There are two methods of knowing whether my friend is alive: (M1) knowing he was alive yesterday and continuing to hold this (highly probable) belief; (M2) seeing him alive right now. If my friend were dead, then I could not know anything from M2 and would believe a falsehood of his being alive through M1. But M2 outweighs M1, such that if I were to see him dead, I’d believe that he was dead, despite knowing that he was alive yesterday. And, of course, if I were to see him alive, I’d believe that he was alive. So, condition (3) is satisfied through M2: the evidence of seeing is more convincing than the evidence of inference from a trend (it’s a good bet under normal circumstances that a person will stay alive tomorrow if he is alive today), and (3) includes reference to this more powerful method of knowing.
Example 2. There are two methods by which a father believes his son to be innocent of a crime: (a) he has faith in him and his righteousness; (b) his trial has acquitted him. Suppose that the trial has instead correctly proved his son to be guilty, would the father still cling to his now false belief? If so, then he does not have knowledge that his son is innocent, despite the actual fact of (a) and (b), because condition (3) is unsatisfied: p is false in some close possible world, yet the father nonetheless believes that p. (a) outweighs (b) for the father, yet we presume incorrectly. Certainly, the outweighing may be correct, such that the father’s faith is true despite the trial’s verdict, in which case the father would have the knowledge of his son’s merits or demerits.
Example 3. Again, we have two pieces of evidence that a certain building is a concert hall: (1) S has been there and seen the plays; (2) S has read government reports that it is a concert hall despite the fact that it really is a nuclear weapon building factory. (No one can approach the alleged hall because of the radiation which affects a person such that he loses any interest in buying a ticket.) Condition (3) seems to be violated, because in the second case the building is not a concert hall, yet S holds the false belief that it is. Must we deny the relevant knowledge to S? Nozick writes that not all the methods of coming to know something need satisfy conditions 1-4, only the weightiest one. It seems that determining which method outweighs all others must be done on a case by case basis with the help of practical wisdom.
Thus, (3) says that S must possess truth-sensitive evidence for p, such that the falsehood of p would invalidate the evidence. The evidence must “track” p’s truth value. In other words, the evidence must be seen to immediately cease to be good if p is imagined to be false (because, for example, it would be different due to a different method of gathering it). (4) says that good evidence must be available in any situation in which p is true. Nozick’s criteria for knowledge appear to mirror my own (independent) ideas, namely, that there can exist no justificatory evidence for a false belief and that there must exist (though obviously not such as to be always discoverable) evidence for any true belief.