 Just as we can be empirically justified in believing a false proposition (e.g.,  9b:  two quarts of water plus two quarts of carbon tetrachloride do not combine to yield four quarts of liquid), philosophers argue that we can also be a priori justified in believing a false proposition. Perhaps Kant was a priori justified in believing that every event has a cause. He thought that the proposition had that status. Yet many physicists believe that there are genuinely random events at the subatomic level, and reasonably believe it false that every event has a cause. You might initially be a priori justified in believing that no matter how happiness has been produced it is intrinsically good  (12a),  or that it is always wrong to punish an innocent person  (13a).  You might later think of counterexamples to such claims (e.g., by considering happiness produced through the suffering of others or punishing an innocent person to prevent some evil men from punishing him and many other innocent people). Then your initial a priori justification would be defeated. John Hawthorne notes how even paradigmatic instances of a priori justification, such as someone’s carefully working through a mathematical proof, can be undercut if the person gets empirical evidence that he is mad or that his proof is probably mistaken (based on evidence provided by expert testimony, a bad track record, or of distorting background conditions, etc.) (Hawthorne 2013: 2009). These examples seem to show that a priori justification is fallible and defeasible (i.e., it can be defeated by further a priori or empirical evidence).