The difference between a calm and a violent passion is the degree to which the passion is felt. This is based on situations and circumstances. “’Tis remarkable, that lively passions commonly attend a lively imagination. In this respect, as well as others, the force of the passion depends as much on the temper of the person, as the nature or situation of the object.” (pg 273, 2.3.6.9)

“Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.”
I believe it is Hume’s intention in this argument to express his opinion that an individual person’s reason is based on that person’s disposition. His passions are toward one end or another, and as a direct result, his reason can be toward one extreme or the other. This could also be affected by his situation.
Although this statement was made by Hume during the 19th century, we see it holding true even today. An example provided by another writer (Doris, I believe) discussed in this class says that a person who is inclined to be of aid to others may or may not stop to help someone who has spilled the contents of his briefcase on the floor. Despite the person’s inclination, the situation may or may not warrant the assistance being provided.
Another good example would be the Kitty Genovise incident. Although people in the surrounding buildings and those passing nearby could hear Kitty screaming for help, each believed that someone else would provide assistance. Each person’s reason told them to pass and leave the action to someone else. These people’s particular passions were likely not to jump to another’s aid.

“There is not in my opinion any other natural cause, why security diminishes the passions, than because it removes that uncertainty, which encreases them. The mind, when left to itself, immediately languishes; and in order to preserve its ardour, must be every moment supported by a new flow of passion. For the same reason, despair, tho’ contrary to security, has a like influence.” (pg 270, 2.3.4.8)
I am not sure I entirely understand what Hume is saying here.
“One can consider the clouds, and heavens, and trees, and stones, however frequently repeated, without ever feeling any aversion. But when the fair sex, or music, or good cheer, or anything that naturally ought to be agreeable, becomes indifferent, it easily produces the opposite affection.” (pg 271, 2.3.5.4)
“Nothing is more capable of infusing any passion into the mind, than eloquence, by which objects are represented in their strongest and most lively colours. We may of ourselves acknowledge, that such an object is valuable, and such another odious; but till an orator excites the imagination, and gives force to these ideas, they may have but a feeble influence either on the will or the affections.” (pg 273, 2.3.6.8)
I find these two quotes to be particularly interesting. The first provides, at least in my mind, an extremely compelling support for the argument addressed. The second provides a very valid point concerning the ability of skilled speakers to sway the views of others. Not only do I believe this is true, I find this section to be exceptionally well written in terms of pleasant and persuasive wording.