Sensations are things like tickles, itches, pains, felt cold and heat, etc. The occurrence of a sensation is the most fundamental modification of consciousness, and shares all the problems of that concept. It is usually thought of as a primitive inner event distinguished only by its raw feel to the possessor. Sensations are distinguished from thoughts and perceptions, which carry a content with them, although the relation between the bare occurrences that make up sensations and these cognitive states is intensely debated. The private language argument of Wittgenstein is one attempt to overthrow the idea that our knowledge of our own sensations is private, infallible, and a foundation of all the rest of our knowledge. See also perception, qualia.