Skills are more complex than attributes or capabilities. Contrary to how it is for attributes, not all entities possess Skills. Skills are limited to sentient entities (or entities made sentient through different means, be it magic or technology...).
A Character possesses an undefined number of Skills. Those Skills are define at the Type level. They represent basic Skills of an Entity of a given Type. It is possible for a Character to learn new Skills or develop old ones. Characters can however only learn a certain number of Skills at full efficiency. This number is given by a function of his/her/its memory attribute. Full efficiency of a Skill means that the Character is able to potentially use the Skill at its best degree of performance. If the Character tries to learn more Skills, more ancient ones will become "faded". They won't be in immediate memory anymore and the Character won't be able to use them with maximum efficiency. However, after a couple of tries and/or practice, the Character may regain full efficiency in a Skill. We don't think that one can really forget a physical Skill. One might however forget unused knowledge.
Each Skill should be based on an attribute. The Skill will work as a modifier to the attribute to enhance the performance of the Entity. We can also think that if an Entity doesn't have the necessary Skill to perform a given action, it can optionally default to the appropriate attribute. However, not all actions can use the default attribute or if they do, they should do so with an important penalty as it is the case for knowledge Skills.
Example 1. Mathematics
Example: if you haven't learn mathematics, it will be very difficult for you to solve a given problem. However, a very clever person could devise a way of solving the problem without the necessary knowledge. This is however highly unlikely and difficult to do. We'll explain more precisely in another document how Skills will work, especially regarding the use of attributes.