One of the most central doctrines of Hume's philosophy, stated in the very first lines of the Treatise, is his notion that the mind consists of its mental perceptions, or the mental objects which are present to it, and which divide into two categories: impressions, and ideas. Hume's Treatise thus opens with the words: 'All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS." Hume states that "I believe it will not be very necessary to employ many words in explaining this distinction" and commentators have generally taken Hume to mean the distinction between feeling and thinking.[51] Controversially, Hume may regard the difference as in some sense a matter of degree, as he takes "impressions" to be distinguished from ideas, on the basis of their force, liveliness, and vivacity, or what Henry Allison calls the "FLV criterion" in his book on Hume.[52] Ideas are therefore "faint" impressions. For Hume, impressions are meant to be the original form of all our ideas, and Don Garret has thus coined the term "the copy principle" to refer to Hume's doctrine that all ideas are ultimately all copied from some original impression, whether it be a passion or sensation, from which they derive.[52][53]