So far, I have distinguished between perception and factual knowledge. Perception is an awareness of what is, while factual knowledge is a knowledge that an affirmation is true. The final form that knowledge can take is theoretical knowledge. Theoretical knowledge is a knowledge of why something is true.
A set of true affirmations (factual knowledge) does not necessarily explain anything. In order to explain something, it is necessary to state why these truths are true. An explanation is required. This is what theoretical knowledge is. All theoretical knowledge must explain why some affirmation is true. For example, we know that hydrogen and oxygen spontaneously form water when a spark is introduced to the mixture. An explanation of this fact is a part of theoretical knowledge.
Theoretical knowledge does not necessarily explain everything. If the explanation of the formation of water includes the properties of hydrogen and oxygen, then that explanation is sufficient. While we might also wish to know why those properties exist, that is a separate question. Theoretical knowledge as such is possible even if the facts used to explain the fact under consideration are not themselves explained.
Skepticism about this kind of knowledge is a skepticism in two respects. First, we might be skeptical about whether or not any explanation (or kind of explanation) could ever be good enough to explain what it is attempting to explain. For example, a skeptic might suggest that certain scientific results do not entail what they attempt to explain. Water boils at 100 degrees Celsius. We know that this occurs because of the weak bonding properties of water molecules. But it is hard to show that these properties entail that conclusion. So some doubt is possible. Second, we might be skeptical about whether or not our explanations are properly grounded. We assume that this one fact explains the other fact. But maybe these facts are just common causes of another fact. Perhaps we have reversed the correct order of causation the boiling of water is what explains the bonding properties of water molecules. We also have to consider the previous skepticism about both perception and factual knowledge.
These three forms of knowledge are related. Perceptual knowledge does not require any knowledge in order to be had. All that is required is some senses including those of memory, self-awareness, and the five well known senses. From these, various factual claims are justified. So factual knowledge depends on perceptual knowledge. From factual knowledge, theoretical knowledge uses some factual claims to explain others. This means that perception is the lowest form of knowledge and theoretical knowledge is the highest form. It also means that knowledge as it is currently used is not spoken of equivocally. It is spoken of analogously.
Now that I have distinguished between the various forms of knowledge and their associated skepticisms, it is useful to distinguish knowledge on the basis of what the purpose of that knowledge is. This enables knowledge to be understood.