Many people share the intuition that in order to know what pineapple tastes like, one must have tasted pineapple. This is a special instance of the more general intuition that there are truths which cannot be understood unless one has had certain experiences. The influence of this intuition is widespread; it underlies debates about concept acquisition, the individuation of sense modalities, and our knowledge of the external world and of other minds. In this dissertation, I examine a famous argument in which this intuition plays a prominent role, Frank Jackson's knowledge argument. I articulate and defend this general intuition, and offer a diagnosis of its source: the reason these truths cannot be understood by someone who has not had the relevant experiences is that the content of visual experience is partly ineffable, i.e., the content of visual experience cannot be fully expressed linguistically. I distinguish between two types of ineffability, argue that only one of them would explain the central intuition, and isolate a feature of our visual experience that could explain why our experiences are necessarily ineffable. I argue that if the ineffability of perceptual experience is part of an explanation of the central intuition, then it must be interpreted as a claim about the phenomenal nature of experiences, rather than as a claim about the abilities of the subjects of experience. After close examination of three of the most prominent phenomenological features of perceptual experience - richness, fineness of grain, and determinacy - I argue that only determinacy is an essential property of phenomenological content, and thus the right type of property to explain why experiences are necessarily ineffable.