In Section 3, Hume explains how memory is more "tied down" than imagination, without the same power of variation. This, Hume argues, is self-evident and does not warrant further exposition. This aspect of Hume's epistemology is discussed in 'Hume’s Theory of Imagination' by Gerhard Streminger, Hume Studies Volume VI, Number 2 (November, 1980). I particularly like this paper as it uses the word "propaedeutic" which I am fairly sure you, like me, will need to look up.
Section 4 is titled "Of the connexion or association of ideas" and is where Hume begins to flesh-out his theory and place further limits on the the extent of human knowledge. Ideas become connected or associated in the mind through three main routes: resemblance, contiguity in time or place and cause and effect. Of these, the strongest and most extensive, is cause and effect, which is going to be the subject of further examination.
At the end of Section 4, Hume describes the three categories of complex ideas which arise through the three forms of association described earlier, these are: relations, modes and substances. They are then described in more detail in Sections 5 and 6.
According to Hume, there are seven different types of (philosophical) relation:
- space and time
- quantity or number
- degree of some quality, such as weight or colour
- contrariety, such as existence and non-existence
- cause and effect
Part 7 discusses abstract and general ideas and examines "whether they be general or particular in the mind's conception of them." The conclusion is that "the mind cannot form any notion of quantity or quality without forming a precise notion of the degree of each". Hume therefore rejects the notion of abstract ideas. This section is a little complicated. For further discussion I recommend 'Abstract General Ideas in Hume' by George S. Pappas, Hume Studies Volume XV Number 2 (November 1989). To quote, "The main contention of the paper is that the rejection of abstraction and abstract general ideas lies at the very heart of the philosophy of Berkeley, and that pretty much the same may be said for Hume. Berkeley’s defense of a kind of idealism stands or falls with the success of his attack on abstract general ideas, and Hume’s critique of infinite divisibility in matters pertaining to space and time, along with his destructive critique of various metaphysical notions, crucially depends on the successful denial of abstract general ideas."
This takes us to the end of Part 1 of Book 1 of the Treatise. I will continue with Part 2 in my next update.