1. Much of the reasoning of many philosophers depends on a distinction between substance and accident, and these philosophers think that they have a clear ideas of substance and of accident. But the author asks "whether the idea of substance be deriv'd from the impressions of sensation or reflection?" If it is an impression of sensation, then it must be associated with one of the senses, e.g., "if it be perceived by the eyes, it must be a colour." But in that case, substance would be a color (or one of the other impressions), which none of the philosophers just mentioned would allow. [Strictly speaking, the idea of substance would have to resemble the corresponding impression of sensation, and it would refer to the object of the impression, such as a color.] This leaves only impressions of reflection as the source of the idea of substance, "if it really exist." However, the latter kind of impressions are passions or emotions, "none of which can possibly represent a substance." The conclusion that must be drawn is that "we have . . . no idea of substance, distinct from that of a collection of particular qualities. Thus, when we talk or reason about substance, we are only talking or reasoning about a collection of particular qualities."

2. In this paragraph, the author examines the idea we have of substance, and in the next our idea of mode. In both cases, the idea is about a collection of qualities. [From the examples given, it is clear that the author has in mind substance and mode as general kinds of things: gold, a dance, beauty.] The ideas of these qualities are united by the imagination and are given a name, which allows us to recall that collection of qualities to ourselves or to others. The way in which this takes place is very different in the cases of substances and modes. Substances are supposed to be composed of qualities but to be more than just the sum of their qualities. One view [Locke's] is that the qualities making up a substance are "referr'd to an unknown something, in which they are suppos'd to inhere." This view is a fiction. A more modest view is that the qualities are "suppos'd to be closely and inseparably connected by the relations of contiguity and causation." On either of these views, we may begin with a certain collection of qualities which are connected in a certain way, and then add to the collection new qualities which we discover to stand in the same connection. We might at first have an idea of gold as which is a collection of a yellow color, a certain weight, the ability to be shaped [as into rings: the property of malleability], and the ability to be melted [fusibility]. When it is discovered that something with these qualities can also be dissolved in aqua regia [a mixture of hydrochloric and nitric acids], that quality is added to the others and supposed "to belong to the substance as much as if its idea had from the beginning made a part of the compound one." The reason that a new quality can be added on a par with the original ones is that the "chief part of the complex idea" of a substance is "the principle of union," and not any particular quality.

3. On the other hand, no new qualities may be added to the collection of qualities that makes up a mode. There are two kinds of modes. The first is one in which the qualities making up the collection are dispersed among various objects, as with a dance, the idea of which includes different dancers. So there is no union based on contiguity and causation [let alone an "unknown something"], and hence no basis for adding new qualities. The second kind of mode is one in which the qualities are united in one object, but in such a way that "the uniting principle is not regarded as the foundation of the complex idea." Beauty would be an example of a mode of a second kind. [The union of the qualities that make a person beautiful is not that which makes the person a substance. There is nothing about the former union which admits of allowing new qualities to be added to it in the way that new qualities can be added to the object qua substance.] If a new quality were to be added to the complex making up the mode, we would be talking about a different mode, which would have to be given a different name.