Here Hume is arguing by example that people who do not have the appropriate impressions (premise 2), do not have the corresponding ideas (premise 1). When these people do get the impressions (premise 4), then they have the ideas (premise 3). So these ideas, at least, seem to be dependent upon impressions.

The remainder of the argument repeats this logic but concerns not people who are blind or deaf (as in premise 1), but people who simply have not had the occasion to have the appropriate impressions (premises 5 and 6). (Hume presumes that a "LAPLANDER or NEGRO" [text] has not tasted wine, but these particular examples are not necessary to the argument, of course.)