Hume

Hume suggests that the mind has an incredibly limited understanding, if any understanding at all, of the workings of nature.  In this light, we are essentially powerless in regards to the workings of nature .  He supports this idea with the problem of induction, that the future actions of certain products cannot be determined by past experiences, as these experiences determine the probability of a certain event, but cannot make a absolute determination as to whether or not an event will occur.  Despite this he does not advocate the abolishment of practical behavior, as practical behavior is necessary for survival regardless of the truth behind the action.  In this regard the problem of induction has no consequence over those who simple act without much thought, but for those who require reason behind their action, the problem of induction becomes a hindrance.

It is from this line of thought that Hume advocates the abolishment of the study of and belief in metaphysics.

He makes the statement that one cannot dream of something without first having experienced it.  To Hume all thought is a product of experience, so how can we determine anything about what is outside of our experience? As soon as something metaphysical becomes experienced, it is no longer metaphysical, as it has subjected itself to the physical means by which a human needs in order to have an experience.