 Although this particularist approach probably is the way in which most philosophers think of certainty, it faces several difficulties. One is that the epistemology of the a priori is far from clear. Given that we do not, apparently, causally interact with necessary truths, it is hard to see how our minds can have access to them. A second difficulty has to do with knowledge of our own mental states—sometimes referred to as knowledge by acquaintance. According to the “speckled hen” problem, there are aspects of our mental states, such as the rich detail of one’s present visual experience, that we are not capable of knowing—e.g., if one is looking at a speckled hen, there will be a determinate number of speckles in one’s visual experience, which one will not be able to know just in virtue of having the experience (Ayer 1940, Chisholm 1989, Fumerton 2005). But those aspects we cannot know merely by being conscious of them are part of our conscious experience in just the same way as those aspects we are supposed to be able to know; the difficulty is specifying a principled difference between the two. Much more could be said about the first two problems, but they lie beyond the scope of this article. A third difficulty is that, at least prima facie, knowledge of one’s mental states seems to be of a fairly different kind from knowledge of necessary truths. It is not clear, at the outset, that we are warranted in taking them to be paradigmatic instances of a genuine epistemological kind.