In his Treatise on Human Nature he declares that man's mind is only a collection of perceptions. These perceptions are either impressions or ideas .
Impressions are sensations of pleasure, pain, awareness of qualities and relations.
Ideas are but the faintly remembered images of impressions formerly experienced. If things outside us really do exist, there is no proof of their existence available to us. What a man needs, said Kant, and what he can have is truly scientific knowledge , that is, knowledge that is universally and necessarily true and reliable. The experiences of the senses is individual, and, no matter how consistently and for how long a time the senses find a fact solid, there is always the possibility that the next experience will show it to vary. But, said Kant, there is another element in knowledge, an a priori and subjective element which is anterior to sense-experience and in no wise dependent on it.