In this section of An Enquiry concerning Human Understanding, Hume attempts to create an outline for necessary connection and causation, and provide an apt definition for cause.
Hume begins with the claim that mathematical sciences have an advantage over the moral sciences. This advantage is that the ideas in the mathematical sciences are “always clear and determinate” (Enquiry VII, 1). For example, an oval is clearly distinct from a circle, but virtue and vice (i.e. right and wrong) are not clearly distinct from one another (VII, 1). Hume claims that moral ideas introduce ambiguity in our reasoning (VII, 1). Terms such as power, force, energy, or necessary connection are obscure and uncertain. Hume wishes to understand and remove the obscurity surrounding those terms. He argues that all ideas and complex impressions are initially formed by simple impressions (VII, 4), but even simple impressions have some ambiguity (VII, 4). In order to understand simple ideas, we must be able to trace them to where they are derived (VII, 5). Hume argues that there is no simple impression that could inform us of necessary connection.
Hume examines our impressions of interactions between two bodies (i.e. objects), between mind and body, and within the mind. In each case, he argues that we cannot perceive any impressions of necessary connection. Firstly, Hume discusses the body-body interaction of billiard balls. He states that we can observe that when one billiard ball strikes another, the motion of the first billiard ball is followed by the motion of a second ball. However, we are unable to observe the act of causation (VII, 6). He argues, also, that our mind is unable to perceive the workings of cause and effect. If that was the case, we could determine what effects would follow from causes without needing to observe anything (VII, 7).
Next, Hume examines the interaction where an act of volition can cause the movement of limbs. He points out that even though we are aware of our ability to move our body, we are not aware of the connection between our will and the ability to move our body. Hume claims that the connection between body and mind is poorly understood, since we cannot understand why we are capable of controlling our fingers and tongue, but not our heart or liver (VII, 12).
Finally, Hume examines mind-mind interactions. He states that we are unaware of how the mind can conjure an idea out of nothing. Through experience, the mind has varying degrees of control, so that it has more power over reason than the passions (VII, 18), or that it has greater self-command when it is healthy (VII, 19). Since we learned these things from experience, we are observing only a constant conjunction and not a necessary connection (VII, 21).
Hume continues to examine that some people will acknowledge that what we perceive as causes are in fact occasions and the God is the ultimate cause of all change (VII, 21). Hume goes on and says how this claim, that God is the ultimate cause of all change, is used by many philosophers as the conclusion to all the things he questioned earlier. That is, God is the immediate cause of the connection between the interactions of two bodies, the mind and body, and within the mind. Hume is unable to understand how philosophers reach these conclusions, which he thinks are illogical and unsupported (VII, 24). Furthermore, he questions how we can perceive the forces that are operated by the mind of God if we cannot even comprehend the forces that operate our own mind and body.
Hume sums up by creating two definitions of cause. His first definition, which defines a cause as “an object, followed by another, and where all objects similar to the first are followed by objects similar to the second” (VII, 29), accounts for all the external impressions involved in the case. His second definition defines a cause as “an object followed by another, and whose appearance always conveys the thought to that other” (VII, 29).
Hume’s argument in section VII of the Enquiry is very intriguing. Considering body-body, mind-body, and mind-mind interactions, Hume is able to show that there is no evidence of a necessary connection. If we knew of necessary connection purely through reason, we would not need experience to show us that two events are necessarily connected. However, in each case, Hume is able to show that it is the experience that teaches us this connection. Moreover, we do not actually experience a necessary connection; we only infer from the constant conjunction that we repeatedly observe given a situation. For example, in every single case where one billiard ball strikes another, we observe the movement of the second billiard ball as soon as it strikes the first one. This observation leads us to infer that there is some connection between the collision and the movement of the two balls, even though we are not able to observe that connection.
However, Hume’s conclusion of causation is a belief in matters of fact. His conclusion is neither logical knowledge nor mathematical knowledge. Essentially, Hume’s conclusion is based on the operating mechanism of the mind. That is, it is based on past experiences regarding a particular interaction of two entities (i.e. I can infer what will happen once I see those entities interacting in the same way again). But according to Hume, such a belief in matters of fact is always contingent (i.e. that is, it is true in some possible world). It would then follow that his conclusion is also contingent, and cannot be justified by reason. Therefore, given his intriguing take on causation, Hume ultimately contradicts his own conclusion.