In morals, politics, religion and science, Hume was a conservative empiricist who emphatically rejected all theories he thought of as metaphysical or not based on actual experience and sense perceptions. He did not regard religious and metaphysical theories as scientific, but more like idle speculation, superstition and prejudice. No ultimate original principles existed outside of the mind and perceptions, and this certainly included the concept of cause and effect, which he insisted was derived from the senses and later processed through the mind in the form of simple and complex ideas. Nothing could be known about human nature or any other subject outside of an exact, empirical science, while innate and a priori ideas did not exist. Even his theories of mathematics, logic and the color spectrum were all based on empiricism, and the ability of the mind to reflect, compile and make connections based on repeated sense experiences. In short, he had no use for all the complex system building of the Continental European philosophers, although his rigid empiricism risked carrying him over to the opposite extreme and reaching peculiar conclusions, such as doubts about whether physical or mathematical laws were actually operating independent of the observer. Hume never intended to be a nihilist or subjectivist, since he agreed that a real physical universe existed to be perceived, even though he denied that human beings could have meaning meaningful knowledge about its ultimate causes. His theory did not lead to outright denial that nothing could be known about the world at all, but it did make him skeptical about any universal theories or laws about gravitation, motion, energy and other forces. Hume preferred to keep his theorizing closer to common, everyday experience on earth, and to causes and effects that could be easily verified by any observer on this planet.
As an empiricist and insists that human beings have no innate ideas but only information from sense impressions, which is why metaphysics appears to be incomprehensible. Simple ideas are processed through mental faculties like the imagination to form complex ones, but they only exist because of "the materials afforded us by the senses and experience" (Hume, p 11.). As an example of this Copy System, Hume gives the example of the person who has seen every shade of blue except one. When he is shown a chart of colors with a blank spot where that shade would be, his imagination would be able to create an idea of what this color would look like, even though he had never seen it. All complex ideas can ultimately be analyzed and broken down into the simple ones that are their original components, and if they cannot be then they are meaningless. All ideas are connected by association, a principle that he considered his most important contribution to philosophy, and three of the mental faculties or functions that occurred under this category were resemblance, contiguity and cause and effect. Causation was the most powerful mental function of all only the only one that is "beyond the evidence of our memory and senses" (Hume, p. 22) For Hume, association was as vital to philosophy and psychology as Newton's laws of gravity were to physics, and it made a true science of human nature possible.
Hume denies that the concept of cause and effect exists a priori, or that causal reasoning is innate. No inexperienced thinker would be to make connections of cause and effect, even in mathematics and other abstract subjects, which are based on the study of laws and theories that is also learned through experience. Without prior experience of similar events no one would be able to make any causal connections at all just through logic, intuition or reason alone. In this sense, mental functions are basically programmed to expect that future events will generally be like those in the past. This is how infants, children and even animals learn though experience and no elaborate or intricate theories are required to explain it. For example, children learn through experience that water is wet and fire is hot, and that putting a hand in a fire will cause pain. Beliefs about cause and effect were not rational or cognitive but based only on information derived from the senses.
Cause and effect is based on a mental habit of associating the connection and conjunctions between separate events, essentially a function of the mind that is learned over time. This causal connection comes because "after a repetition of similar instances, the mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist" (Hume, p. 50). The mind remembers that such events followed each other in the past and infers that this same pattern will be repeated in the future. By Hume's definition, the associations and memories build up because similar effects follow similar causes repeatedly, and end up conveying the same thought or idea in every case. For Hume, causation and other major philosophical problems can never be solved with a priori ideas, and only experience firms the basis for comprehending cause and effect. Indeed, he goes on that all morality and human behavior is based on this same principle. He is always skeptical of broad, overarching metaphysical theories and explanations that are not based on actual experience, and this is also true in politics, religion, history and ethics.
All beliefs and opinions were based on sense impressions and the ideas derived from them, and the stronger or more vivid the perception, the more impact it had on the observer. Hume was not a subjectivist or idealist in the sense that he doubted the existence of a real physical universe outside of the mind and its sensory apparatus, but only denied that anything could be known about it apart from sense perceptions. Impressions are rejected or accepted as true based on their coherence and compatibility with earlier ideas and experiences, but he refuses to speculate on any of the ultimate causes in nature or the physical world that might give rise to sense impressions. A Humean scientist would therefore base his theories only on that which people can see, hear, feel, taste and smell, and that all theories about the universe are subject to continual updating and revision. Human beings make inferences and inductions about the world out of habit, which serves them well enough when the future is similar to past experience. Newton also argued that theories derived from inductive reasoning were usually true, until disproven by more accurate ones based on future research.
For Hume, however, Newtonian forces like gravity are not real or ultimate causes, but only based on ideas about effects. His philosophy has no final or metaphysical causes or any concept that the universe has a Designer or Creator, as Newton believed. He concerns himself only with efficient or immediate causes, readily perceptible to the observer, and knows nothing about the existence of any other kind. Nor would he ever be able to state that a force like gravity is always the same throughout the universe as people can perceive it on earth, since they obviously lacked the ability to travel to other stars and planets and observe it directly. He denied that a cause could occur simultaneously with an effect as well, such as the acceleration of an object due to gravity.
Hume's understanding of science has more in common with that of Aristotle and the other pre-Newtonian philosophers, although he was also aware that colliding objects could be both causes and effects to each other. In general, he held that causes always preceded effects, that they had to be evident to an observer in the same space and time, and the same cause would always produce the same effect. Only though repetition and habit did people even develop any notion of cause and effect, and if the observed effects did not resemble each other logically they must arise from different causes. Hume did allow for compound or complex causes and effects, but these followed the same rules as simple ones, and that effects must be proportional to the cause. If the former is diminished in power or intensity in some way, then so must the latter. He could not accept universal laws of gravity, however, including its effects on tides, comets and the movements of stars and planets, which he claimed was based on an "arbitrary supposition" (Hume, p. 26). His theory of cause and effect can only extend as far as earthy examples known to human observers on this planet. Hume is therefore not a natural philosopher or scientist in the modern sense, at least not fully, since he cannot accept the extension of Newtonian causality and forces throughout the known universe. His science is more limited, cautious and conservative, skeptical of wide-ranging or overarching theories that cannot be empirically verified. He allows only ideas based on distinct sense impressions, the stronger the better, and is more careful about revising traditional…