I have reduced the types of knowledge down to experiential, factual and theoretical knowledge. Experiential knowledge is a knowledge of particular things gained by perception. These two elements are what make experiential knowledge what it is. This also means that skepticism takes the form of denying that perception is successful in some way.
Experiential knowledge is the knowledge gained by experience. It is always a knowledge of particulars. Particulars are individual things, events and characteristics. If someone were learning to ride a bike, a particular might be how he moved his muscles while riding that particular time. It would also be what he saw while riding. It would also be any other particularity of any kind. All knowledge of particularities is experiential knowledge and all experiential knowledge is a knowledge of particularities.
Experiential knowledge is identical with knowledge gained by perception. Through perception we come to know particular things. We see them, hear them, taste them, etc. This claim is self-evident. Simply consider any particular thing at all and consider how knowledge of it is obtained. It is always obtained first by perception. We might have (via testimony) the knowledge that a particular thing has particular characteristics, but the testimony came from someone who first perceived it. That experiential knowledge, knowledge of particulars and knowledge by perception are all the same can be proven in several ways.
First, perception is always the perception of particular things. We never perceive redness in general or hear a generalized sharp sound. We always see redness as it appears in that situation and that kind of sharp sound. Second, perception of particular things always gives knowledge about them. So when we see or hear something, then we always know something new about it even if it is only that it appeared that particular shade of red one time. Therefore, perceptual knowledge is always a form of experiential knowledge. Third, experiential knowledge cannot be gained without perception. Experiential knowledge is always knowledge of how a particular skill of thing works in the various particular situations of real life. Those situations cannot be truly known unless they are known perceptually.
Knowing this, there is a distinction to be made between perceiving that a thing is red and redness appearing to one’ s senses. The first is a case of successful perception. The second does not claim success. If I look at a bright light, then I will see red and blue spots in my vision. Even though red and blue is appearing to me, there is nothing in reality that I am actually seeing. Although I cannot doubt that I am seeing red and blue spots, I can clearly doubt that there is anything real. Skepticism in perception amounts to the claim that certain types of things or all things might be an illusion. This possibility should give rise to some form of perceptual doubt. Maybe I should doubt that I am actually successfully seeing anything at all. The nature of the doubt and the form that the doubt takes all depend on what kind of skepticism is in view.