Humes biggest contribution to philosophy was his attack on the foundation of knowledge itself.  By questioning the basis upon which we draw conclusions about the world from experience, he demonstrated how weak this foundation really is.  For example, we assume that events relate to each other as in a cause and an effect such as one billiard ball hitting another and causing the other ball to move.  However, all we really see is one ball move and then the other ball move.  We infer that one caused the other yet we never see the cause.  We have no basis to infer that the second ball must move because the first one hit it.  In fact, we just assume the two are related because we have seen the events happen together so many times.  In fact, whenever we see two things happen closely together, the human mind associates them as a cause and effect but this is unwarranted.  The attack on induction gets worse.  First, we make a number of observations of nature such as a dropped object falling to the ground. Second, we see this many times.  We conclude that a released object will always fall to the ground.  However, there is a built in assumption which is that nature is uniform or will behave uniformly.  If this is true, then we can conclude that past events will be repeated in the future, i.e. a released object will fall. However, we have no basis to prove nature is uniform.  Therefore, we have no basis upon which to believe that a released object will fall to the ground.  Worse,  all inductive arguments have this flaw.  We do this from what Hume calls "sentiments" or just a habit of mind.

 

Many attacks on Hume's arguments have been made but none decisively answer this problem.  In my opinion, this argument should not be seen so much as the defeat of philosophy or reason but a boundary on absolute knowledge.  We cannot get through the day without induction based on experience. Even Hume had to do this and in the end, we survive with this approach.  However, we must accept that we can only get probable answers in this manner, not foolproof answers.