The difference between the two meets in the degree of force vivacity, that is a natural index and practically unequivocal of differentiation it enters the two types of perception, being that the ideas are taken by it as weakest and weak, copies of the impressions in the memory or the imagination and the impressions are stronger and alive, mentioning always the sensations, passions and emotions. In the Inquiry Concerning the Human Agreement from a reflection deepened of understanding that the capacity to distinguish between ideas and impressions could until serving to discern between normality and illness, between health and madness.
Being that, the common degrees of these qualities (the force and the vivacity of the impressions and ideas) are easily distinguished, even so are not impossible in certain particular cases approach one them others. A practical example on that is when under sleep, the fever, madness, or any very violent emotions of the soul, our ideas can be come close to our impressions. The force and the vivacity mark for characteristic us that they make in them to distinguish of course, and in normal circumstances, feeling of thinking. It is possible to confuse impressions and ideas if due to some type of clutter or riot one to acquire extreme vivacity and force. E, inversely, certain impressions can be so weak the point of being taken by ideas. This occurs because the difference of force degrees does not imply a ontological distinction between our perceptions, that is, for Hume that idea that I have of the red formed in the dark one and the impression that reach our eyes under the light of the sun differ only in degree and vivacity, and not in nature.. For more information see this site: Peter Kern.