Hume asserts that the compatibilist picture he formulated in Part 1 of section VIII is both consistent with and essential to our common understanding of morality. Our behavior is guided by motives according to which we pursue rewards and avoid punishments. Hume suggests that we might look upon these motives as causes for our actions, following seemingly necessarily from our characters and natures. We blame people not for bad actions, which are fleeting, but for the motives and causes that lead them to act badly. People are considered wicked if their bad actions seem to be motivated by a bad necessity or cause that springs from their nature. The libertarian incompatibilist that denies necessity must thereby deny also our sole criterion for praise or blame. Similarly, the hard determinist that denies free will must deny that our actions are rooted in our characters, which would also render us blameless. Only a compatibilist picture that reconciles free will with determinism can be consistent with the standard assumptions of moral practice.
Next, Hume faces two possible objections relating to God as being the sole author of all deeds. The first objection is that there can be no bad actions in the universe, since a perfect God is the ultimate cause of all actions. Actions that may appear bad to us would in fact appear good if we could see them in the wider context of God's creation as a whole. Hume counters this objection by pointing out that our sentiments of praise and blame come not from some sympathy with God's ultimate purpose but from what promotes peace and security, or disorder and unhappiness, in human affairs. While philosophy or religion might show that everything is ultimately for the best, our moral sentiments are governed not by metaphysical speculations but by the natural sentiments of the human mind.
The second objections is that if we deny that everything in the universe is as it ought to be, we are ultimately placing some blame upon God. Either God's powers are limited, or he is responsible for creating all the evil that is present in the universe. Hume remarks that such an objection is beyond the scope of philosophy. We have enough difficulty understanding the workings of the human mind and common life. To delve into the mysteries of God's will and motives is sure to entangle us in a web of uncertainty and error.
Section IX of the Enquiry is a short section entitled "Of the Reason of Animals." Hume suggests that we reason by analogy, linking similar causes and similar effects. He suggests that his theories regarding human understanding might then be well supported if we could find something analogous to be true with regard to animal understanding. He identifies two respects in which this analogy holds. First, animals, just like humans, learn from experience and come to infer causal connections between events. Second, animals certainly do not learn to make these inferences by means of reason or argument. Nor do children, and nor, Hume argues, do adults or even philosophers. We infer effects from causes not by means of human reason, but through a species of belief, whereby the imagination comes to perceive some sort of necessary connection between cause and effect. We often admire the innate instincts of animals that help them get by, and Hume suggests that our ability to infer causal connections is a similar kind of instinct.
In these pages, we gain a clear insight into Hume's naturalist and anti- rationalist thinking. Rather than follow in the tradition of rationalist philosophy and try to uncover the mysteries of the universe according to a priori reasoning, Hume abandons all pretensions to knowing truths that transcend experience. He does not try to detect the origins of many of our common sense or philosophical notions in the secret workings of the universe, but asks instead how they arise in us.
For instance, Hume's explanation of moral judgments at the end of section VIII is based purely on observations of our natural behavior. We judge something to be good because it promotes happiness, security, peace, or what have you, and we judge something to be bad because it promotes the opposite of what we consider to be beneficial. Hume does not deny that there may be some God with some ultimate purpose and some ultimate sense of right and wrong so much as he denies that this is the origin of our own ideas of right and wrong. Rationalist philosophers such as ##Descartes## and Leibniz worked very hard to reconcile metaphysics and ethics, developing metaphysical systems that explained God's presence and role in the universe, and inferred principles of right and wrong from this metaphysics. Hume concedes that all may be for the best in the universe when considered in the wider context of God's creation, but points out that our understanding of morality is not based on this wider context. In this way and others, Hume steers away from the metaphysical and toward the empirical in examining ethics. Modern ethics, for the most part, has taken this line of thinking, seeing the principles of morality as founded ultimately in human reason and human action rather than in the workings of the universe.
Hume also follows a naturalist line in examining reason in animals. Rather than perceive reason as a special faculty of the human mind that allows us to see into the truth, Hume interprets reason as a faculty that has arisen naturally, one that helps us get by in the practical world. Animals, too, have this ability to reason, and we find it most distinctly in our inferences from causes to effects. As Hume has argued already, these inferences are not rationally justified, but are, rather, useful determinations of thought. In this section, he goes so far as to liken them with instincts. Human reason then differs from animal reason only in the sharpness and precision of our ability to infer necessary connections in nature and to think about them.
We might identify this argument with the skeptical line in Hume's thought. Contrary to rationalist philosophy, Hume argues that our reason is not a truth- tracking device that can a priori understand the many mysteries of the universe. Rather, it is simply a tool that guides us through life. All our higher reasoning is based on perceiving necessary connections in nature, and yet we never perceive any necessary connections that go beyond constant conjunction. Thus, our higher reasoning has no more rational justification than that of animals.