Virtue is an activity of the human soul and has two parts virtue of character and virtue of intellect. Virtue of intellect is wisdom and knowledge an example would be studying at school. Virtue of character is learned by action and practice an example would be learning that cheating on tests is bad and knowing not to attempt it. Reasoning out where we stand in a particular situation is not related to knowledge or studying but rather repeated application of acts so Aristotle’s is stating acts of character not of intellect.
Nothing can be to good, have to little and you have deficiency have too much and it’s excess. The mean is the middle ground between too much and too little. This mean varies on person to person. An example would be too much sun. Take a 2 people one with a fair skin tone and one with a dark skin tone. The mean amount of sun for the dark skin tone would be excess to the fair skinned person and possibly give them sunburn. Give the dark toned person the mean amount of sun required by the fair toned person and you have deficiency and