The "perceptions of the mind" (what we are directly aware of) are of two kinds:

Impressions:	Ideas:
feeling pain	remembering feeling pain
	imagining pain
seeing a face	thinking about a face
feeling anger	thinking about anger

"By the term impression, then, I mean all our more lively perceptions, when we hear, or see, or feel, or love, or hate, or desire, or will. And impressions are distinguished from ideas, which are the less lively perceptions, of which we are conscious, when we reflect on any of those sensations or movements above mentioned." (P 172a)