In An Inquiry Concerning Human Understanding, David Hume demonstrates how there is no way to rationally make any claims about future occurrences. According to Hume knowledge of matters of fact come from previous experience. From building on this rationale, Hume goes on to prove how, as humans we can only make inferences on what will happen in the future, based on our experiences of the past. But he points out that we are incorrect to believe that we are justified in using our experience of the past as a means of evidence of what will happen in the future.
Since we have only experience of the past, we can only offer propositions of the future. Hume classifies human into two categories; Relations of Ideas, and Matters of Fact. (240) Relations of ideas are either intuitively or demonstratively certain, such as in Mathematics (240). It can be affirmed that 2 + 2 equals 4, according to Humes relations of ideas. Matters of fact on the other hand are not ascertained in the same manner as Relations of Ideas. The ideas that are directly caused by impressions are called matters of fact.
With matters of fact, there is no certainty in establishing evidence of truth since every contradiction is possible. Hume uses the example of the sun rising in the future to demonstrate how as humans, we are unjustified in making predictions of the future based on past occurrences. As humans, we tend to use the principle of induction to predict what will occur in the future. Out of habit, we assume that sun will rise every day, like it has done in the past, but we have no basis of actual truth to make this justification. By claiming that the sun will rise tomorrow according to Hume is not false, nor is it true.
Hume illustrates that the contrary of every matter of fact is still possible, because it can never imply a contradiction and is conceived by the mind with the same facility and distinctness as if ever so conformable to reality (240). Just because the sun has risen in the past does not serve as evidence for the future. Thus, according to Hume, we are only accurate in saying that there is a fifty- percent chance that the sun will rise tomorrow. Hume felt that all reasoning concerning matter of fact seemed to be founded on the relation between cause and effect. 241)
Hume said that even though the cause preceded the effect, there is no proof that the cause is responsible for the effect’s occurrence , it could be purely coincidental. He claims that the human notion of cause and effect is ungrounded in empirical evidence, but rather given only reasonable probability through continuous reinforcement. Hume’s rejection of causation implies a rejection of scientific laws, which are based on the general premise that one event necessarily causes another and predictably always will.
According to Hume’s philosophy, therefore, knowledge of matters of fact is impossible, although as a practical matter he freely acknowledged that people had to think in terms of cause and effect, and had to assume the validity of their perceptions, For example, if I touch the hot stove, I will get burnt. This statement does not necessitate that when I touch the hot stove, (cause) I will always get burnt (effect). Instead, according to Hume, I have no good reason to think that it will not happen again.
Hume, however, went further, endeavoring to prove that reason and rational judgments are merely habitual associations of distinct impressions or experiences. Hume claims that all our ideas, which form the basis of our knowledge, are derived from impressions that we take in from the outside world and into the inside world of our mind. Hume grouped perceptions and experiences into one of two categories: impressions and ideas. (238) According to Hume, ideas are memories of sensations but impressions are the cause of the sensation. An impression is part of a temporary feeling, but an idea is the permanent impact of this feeling.
Hume believed that ideas were just dull imitations of impressions. Hume did not believe that a priori, knowledge based on reasoning can deduce true knowledge. Knowledge based on reasoning alone, according to Hume does not provide understanding of the real world. He believed that all ideas have to have impressions, that the human mind invented nothing. So, according to Hume, a priori reasoning does not offer any understanding of the real world, because they cannot be traced to the impressions that first created them. The human mind takes simple ideas, and turns them into complex ideas. 243) An example of this concept is the idea of an unicorn. Unicorns are conceived as being horses with horns.
Humes claimed that an unicorn is formed of two simple ideas, the figure of a horse and a horn. Hume concludes that our beliefs can never be rationally justified, but must be acknowledged to rest only upon our acquired habits. In similar fashion, Hume argued that we cannot justify our natural beliefs in the reality of the self or the existence of an external world. From all of this, he concluded that a severe skepticism is the only defensible view of the world, though he does not expect us to live our daily lives by this notion.
Wesley C. Salmon points out that according to the principle of uniformity of nature that even though we do not know for sure what will happen in the future, we must assume that nature will continue as it has done in the past. This is the human condition, in that we have no way of asserting what will happen in the future. But in living our daily lives, we are better to go by what has occurred in the past in nature, despite Humes philosophy that there is only a 50/50 chance. In order to function, we need to accept that there is a uniformity of nature in order to carry on with our lives.