First off what even is a treatise? A treatise is a systematic, written argument using methodical discussion(s) of the facts, the principles involved, and the conclusions reached. Just wanted to state that for anyone who has never heard the word. Now, to the actual essay reflection.
Perceptions - Primary
Hume begins by distinguishing what he considers the two main kinds of perceptions we have: impressions and ideas.1pg. 9 He distinguishes the two based on the degrees of force/liveliness with which they impact/enter our minds.2pg. 9 The more forceful being an impression, but also that which is our initial impact.3pg. 9 This of course leaves ideas as the less forceful, "faint image" of the perceptions seen in our thinking/reasoning.4pg. 9 Note that ideas and impressions may vary in the subtlest of ways, in so that "we can't distinguish them."5pg.9
Perceptions - Secondary
Hume further divides these two again simultaneously into simple and complex.6pg. 10 Simple perceptions being such that they have a singular presence, whereas complex ones can be distinguished into parts.7pg. 10 Additionally, many of our complex ideas might not be correlated to any impression, and inversely our complex impressions might never be formed into ideas.8pg. 11 In a similiar fashion, simple impressions have corresponding simple ideas and vice versa, though Hume "venture[s] to affirm ... that the rule ... holds without any exception ..."9pg. 11 Hume asserts that simple perceptions differ in degree and not in nature, thus they must have the corresponding perception.10pg. 11
If that sounds confusing here is Hume's example on pg. 11:
Consider the idea of the color red. The visual impression of the red that we might see varies based on light intensity. With that in mind the impressions change in degree and not in nature. Therefore, it remains consistent in its simplicity.
Subject of the Treatise - A General Proposition
The comprehensive examination of how perceptions stand in regards to their existence and relations to determining classification as a cause/effect is the subject of this treatise.11pg. 11 With that in mind Hume proposes this:
"That all our simple ideas in their first appearance are derived from simple impressions, which are correspondent to them, and which they represent."
Hume: The Essential Philosophical Works | Part II - A Treatise of Human Nature | Book 1 - Of Understanding | Part 1 - Of Ideas, Their Origin, Composition, Connexion, Abstraction, etc. | Section 1 - Of the Origin of Our Ideas | pp. 11 -12
Hume approaches creating a hypothetical proof by first conjuring anecdotal simulations. He looks at the order of "first appearances" and not surprisingly finds impressions to take precedence in all cases.12pg. 12 Looking into his proposition on that simple impressions beget simple ideas and never the other way around note the terminology. Impressions are the impactful FIRST perceptions while ideas are the reasoned images of the impressions. So, by Hume's own definition, it must logically be this way.
He continues by noting the scarce few people he believes capable of raising "up to himself the idea of that."13pg. 13 In continuing, though our ideas are images of our impressions we can logically form secondary ideas (images of the primary idea).14pg. 13 This is not an exception to his propositional rule rather "an explanation of it."15pg. 14 This is possible due to the fact that at the origin in Hume's system an impression was the original "fuel" that eventually spawned the secondary idea(s).16pg. 14
Hume starts of with this proposition due to number of disputes in various areas of philosophy that "innate ideas" has caused. He flaunts that "ideas of passion and desire ... have a preceding experience[s] of ... emotions in ourselves" and thus "philosophers do nothing, but shew away that they are conveyed by our senses."17pg. 14 Obviously, a directed message towards most of contemporaries that he believes are doing nothing in their philosophical endeavors, but lying to themselves with false premises.
PS - It took me 3 days to simplify his 5 pg.'s to this.