Well, the most obvious difference between Hume's theories and Descartes's is that Hume is much more concerned with our perceptions and impressions of the world around us, and how those influence all our reasoning. Descartes argues that we cannot imagine anything new. Hume takes this one step further (in the other direction) saying that since all of our imaginings must derive from our impressions of the outer world, all internal thought is derived from those impressions: "all our ideas or more feeble perceptions are copies of our impressions or more lively ones" (p 8). (Which I don't agree with at all. Dreams can seem more vibrant and vivid than the real world, people going through PTSD often magnify those past event until the event overshadows their present life.) Because of this, Hume argues that "it is harder to make mistakes about [outward or inward sensations]" (p 9), while Descartes argues that all sensations should be suspect. Hume states that all knowledge is merely knowledge inferred from past experiences; we do not actually know the causes of anything, we can merely examine and map the outcomes. So all cognition is, in essence, shaky divination.
The interesting part of Hume's ideas to me were his (implied) ideas of time. Although he discusses cause and effect at length, he doesn't really touch on the aspect of cause and effect which occur in time. When he says we "experience" an object, or idea, what he really means is when we see an object interact with other bodies around it. So, in order to understand something we must perceive its interactions with its environment and in order to do that, we must see it in time. An item existing only at time A will interact with nothing, and therefore, will not really provide us with any experience of it, or any effect on anything else. It would even prevent us from interacting with it, as we must move through time in order to examine and process it (so it may as well not exist). However, once it moves from time A to time B, all objects it interacts with in that time will grant us a deeper understanding of its special innate "powers" (as well as the objects interacting with it). And since it must move through time to in fact be perceived by us, I would argue that times A and B have to be connected. It is not just our minds connecting those two events, the object in fact cannot exist if it interacts with nothing, including time. So, its interactions are really just extensions of the object in time. Sorry if this got way too abstract for even me to follow...
"A picture naturally leads our thoughts to the thing that is depicted in it; •the mention of one room naturally introduces remarks or questions about other rooms in the same building; and •if we think of a wound, we can hardly help thinking about the pain that follows it."
What made it easier for me to understand Hume argument he continues to explain the idea use of words that are associated meaning and even other forms of dialects. He didn't give any example but I believe Hume might of been hinting to the romance dialects, Spanish, French, and Italian. They all share a lot of similar words and verbs that are associated with the same meaning and can trigger a image or meaning to the mind. What I trying to argue is that symbols or words can trigger a memory, where we associated something personal or what learn to from past experiences.
“But does it follow…that the same perceptible qualities must always be accompanied by the same secret powers? It doesn’t seem to follow necessarily” (pg 16)
What David Hume is saying is that regardless of whether “act x” has led to “result y” in all previous observations of or attempts at this specific act, it is still not proof enough to ascertain that “act x” will always result in “y”. Although, this process of inference based on past experience is part of how the natural mind works, this repeated perception of result y following act x is still not significant in determining the certainty of the casual effect as absolute truth. Hume brings up an example of bread and the nourishment one receives from it. Just because all the times that one has eaten bread previously and received nourishment does not give certainty to the idea that the same thing will happen the next time he eats the bread. I agree with this idea that past experiences are not substantial enough to determine the results in future experiences because the uncertainty (that Hume brings up regarding receiving nourishment from bread) can be seen in the case of adult onset food allergies. Just because every day for the past who knows how long someone has eaten bread and has received nourishment from it, who’s to be certain that tomorrow he will not develop a gluten allergy and that same bread will no longer be of the same nutritional value to him. Is it likely that it will happen, no. but it can happen and that possibility (no matter how slim) makes certainty regarding such matters of cause and effect to be impossible.