In the Treatise Hume is quite certain that all the mind consists of is impressions and ideas and associations between them. He believed that there is no "self" because we can never have a single impression of the self; instead all we experience is a continuous flow of perceptions that replace one another in succession. Therefore we have no idea of a self, for every real idea must be derived from some one impression, and the "self or person is not any one impression, but that to which our several impressions and ideas are supposed to have reference". However, a quick answer to this would be to quote " Observation collapses the wave function" - that by trying to observe the self, Hume could change it, and so by saying we can not experience it with our senses, with the technology that we have today, not being able to experience something does not matter. For example can we see particles of light without aid? or can we have empirical experiences of imaginary numbers?, of course we can't but they are still relevant to the way we have come to understand the world. In today's terms, being unable to experience something directly does not mean it does not exist. .
His views on association of ideas are equally as determined by allowing only three types of association; resemblance; contiguity; and cause and effect. His observance of the mind was written when he was still quite young, and as such gives a very sparse account of the mind and how it works. By the time he wrote the Enquiries, he was a little less certain to claim that there is no self by writing in paragraph 52. "Is there any principle in nature more mysterious than the union of soul with body" and concluded by saying that proving that there is a soul or self is extremely difficult.
Hume's explanation of Association of Ideas is that there is firstly resemblance, e.g. when we look at a picture our thought lead us to the original; secondly, contiguity in time and space e.