According to David Hume, our knowledge does not come from reasoning or a priori but rather from experiences and we realized that any particular thing is connected with each other. We tend to disregard this because most of our causal judgments are very familiar, and that often, we instantly judge things.
For example, in applied mathematics, using abstract reasoning and geometrical methods in the application of principles, we based our judgment from the laws which have already been formulated to come up with resulting principles of such laws. Therefore, the discovery of the original law was the result of observation and experience and not because of a priori reasoning. Even in our daily activities, we realize that the casual connection of things does not exist because of reasoning but because of the past experiences we have. From these past experiences, we draw our inferences and conclusions and therefore are able to connect things. 
Hume holds that there is no necessary connection between a cause and its effect.
In Hume’s view, “there is no causality among perceptions”. Hume concluded that there is no need for the possibility that there must be something underlying the various perceived qualities. He thereby denied substantiality to immaterial as well as to material beings. Accordingly, its only our impressions and the ideas that appear to be real. Thus, Hume has analyzed the mind into perceptions. He proceeded to the stage of synthesis which is putting these impression and ideas together by offering an explanation in terms of the elements of the mind which are bound together by various principles of association. 
This means that even though we humans perceive one particular incidence following the other, which happens in our day-to-day activities, there really is no necessary connection between these two things. Moreover, what we can solely trust is the knowledge we have which we acquired from our perceptions. Our idea of causation is based on our expectation for certain events to result after other events that precede them, such as those happened in our past experiences. We do not have a concept of cause and effect, but rather that notion that certain objects have always been conjoined and that have been found to be inseparable. 
“David Hume” Stanford Encyclopedia of Philosophy. (26 Feb, 2001).
5 Dec. 2006. < http://plato.stanford.edu/entries/hume/#Necessary>.
Kolak, Daniel. (1998). The Mayfield Anthology of Western Philosophy. Mountain View, CA: Mayfield Publishing Company.
“David Hume” Wikipedia The Free Encyclopedia. 5 Dec. 2006. http://en.wikipedia.org/wiki/David_Hume.