Theory of Mental Contents
One of the main ideas of this book and basically of Hume’s philosophy is essential that every mental content it’s coming from sensible experience, here its where he makes a distinction between ideas and impressions. Impressions are the sensations, feelings or sentiments that we get from the world, this is vivid and forceful. In that sense are the irreducible and fundamental perceptions which are immediate external or internal data.
On the other hand, ideas are imaginative or a recall from memory making them less strong. For example, the true taste of a fig is more powerful than the idea of tasting it. Hume claims that ideas are base on impressions or a mix of this sensation and our imagination. He actually classifies them by compounding, transposing, augmenting, and diminishing. Later on, Hume also mentions the actions of separating, dividing and mixing. Therefore, he believes that thought and reasoning are born from the power to form images and that imagination is a kind of blurred copy of sensations and feelings.
Matters of fact Vs. Relations of Ideas
Hume addresses another focus on the Enquire that he puts against the relation of ideas, the “matters of fact”. We can say that this distinction is between synthetic propositions (or facts that are gain by experience and not deduced by logic) and analytic or logical truth propositions that are established by demonstrations like mathematics. Hume set aside any consideration for the matters of fact to be base in a priori or innate reasoning saying that they are achieved by repeatedly observing and making an inference about causes and effects.
So anything we can say about the world is a matter of fact, and thus can be justified only through experience and can be denied without contradiction. Relations of ideas can teach us so trust about algebra or geometry, but cannot, as some rationalist philosophers would have, teach us about the existence of the world, God or our selves.
Skepticism and Human Belief
This mixture of philosophy and psychology accentuates the skepticism that is discovered in their way of facing the material world, banishing the distinction between our thinking and the world, and insinuating that reality is forever out of our scope. Hume was fully aware of this and said that the skeptical arguments of the philosophy should not accompany us in our daily lives. In fact, knowledge is based on successions of impressions and causal relationships are the basis. Although some elements of this knowledge seem to us to be indisputable, they do not go beyond, the soil of probability. We hope to see the sun tomorrow, but we cannot prove that it will come out. The fact that there are connections between them and how are those connections can only be indicated by experience.
Hume not only aims to show that the causal relationship is not satisfactory but also tries to clarify why we believe it is. This is why Hume will abort the study of human belief. All beliefs are ideas, that is, a weak perception that we live with the intensity of the immediate experience. Thus the problem is specified in the question because of the intensity of some perceptions that we are not receiving immediately, but are the product of our memory or our imagination.
Induction and Causal Necessity
Reason supports the relation between ideas but matters of fact need to depend on the connection of cause and effect that we get through the experience. A priori reasoning can not hold up the causes of effects but Hume postulates a necessary connection that is depended on the uniformity of nature. In terms of ideas, these regularities appear in the form of Spatio-temporal sequences. Thus, by the law of association, the presence of one of these ideas (or their corresponding impression) raises the idea of the other in the mind.
On the other hand, these matters of fact do not offer the degree of necessity of the formal sciences at all and "are not ascertained in the same way since the opposite of any matter of fact is still possible because it never implies a contradiction". Hume discusses that because we cannot perceive this necessary connection connecting events, the question of whether or not they really exist is irrelevant and futile.
Association of Ideas
There is a kind of law of mechanics that can be the base of the movements of ideas, for Hume it can be reduced to an attraction that not only have a remarkable impact in the world but also in our minds. The succession and flow of representations affect the imagination in a way that order and determine while making the shape of the memory, dreams, fantasy, and understanding. All this creates a system, regularity, nature and the object of science. Contiguity, similarity, and causality are the forms of associations that make ideas come together and combine. The similarity will be decisive for the comparison between ideas (as for their formal relationships), as is the case with mathematics; space-time contiguity will be in the field of factual sciences.
To Hume those principles that organize what is given in the system also organize the belief, because that constancy, regularity, and order in succession, push to believe helping the fiction in our minds. We can only know the effects of these associations, the cause remains occult but under these forms, the spirit is naturalized.
Science as Probability
That way of understanding the cause-effect connection as a habit that leads us and authorizes us to expect the uniform repetition of the experience, could not give us full certainty. It leads us to skepticism. It leads to the renunciation of the Platonic-Aristotelian Greek ideal of the universality and necessity of the episteme (true knowledge) when it comes to real science or matters of fact. For Hume, universality, and necessity only fit in the formal sciences (relations of ideas). Real science can only aspire to probability. Hume believes that the result of his analysis of causality is not the end of science, not even the end of belief in science. Luckily, he says, such important things as believing and not believing have not been left by nature in the hands of philosophers. That is, somehow everything will remain the same: the imagination will continue with its fictions, men will continue to believe and, ultimately, the understanding will continue to work that way, necessarily.
The External World
Hume's analysis is going to be equally applied to our belief in the existence of a world independent of our senses. The opinion of the independent and continuous existence of the external world is so deeply rooted in the imagination that it is impossible to uproot it. Since the elements of the world are perceptions and since perceptions do not exist any more than at the moment they are perceived, it is absurd to suppose that objects continue to exist when they are not perceived; believing it is a natural trend, based on memory and our need for consistency.
In this way Hume contrasts his sense of empirical existence and the existence of objects in the sense that the common opinion gives to that expression: independent and continued reality outside the act of perception and tells us that it is intended to guarantee the existence of the object in this second sense on the basis of a cause-effect relationship that cannot authorize us to leave the plane of empirical existence, according to the correct use of the empiricist criteria.
