Consider the following passage from the History of Animals (588a15-89a10):
In most of the other animals, there are traces of the qualities of soul that are more evidently differentiated in human beings. For there are both gentleness and savagery, mildness and harshness, courage and timidity, fear and confidence, spiritedness and trickery, and, with respect to intelligence [dianoia], something like judgment [sunesis], similar in many ways, just as we have spoken of the parts of the body. For some of these qualities differ only more or less with reference to human beings, and so is man in reference to many things of animals. Some of these qualities are greater in man, others are greater in other animals, but in others they differ by analogy. For instance, in many there is art, wisdom, and judgment, so there is some other natural capacity. This is most evident if one considers the condition at the age of infancy. For in infants it is possible to see traces and germs of their future dispositions, since there is no difference, so to speak, between the soul of a beast and the soul of an infant. So it is not unreasonable if some psychic qualities are the same, others resemble one another, and others are analogous. For nature passes little by little from the inanimate to animals, so that this continuity prevents one from seeing a border or perceiving on which side an intermediate form lies.
The same holds for the actions of life. For of plants the function appears to be nothing other than producing another similar to themselves, which arises through seeds, and similarly of some animals there is no function other than reproduction. Therefore, the actions of thse sort are common to all. But if sensation is added, then their lives with respect to sexual intercourse will differ through the pleasure of this activity, and with respect to parturition and the nurturing of the young. Some animals, therefore, just like plants, complete the reproduction of their progeny according to the seasons. Others trouble themselves about the feeding of the young, but whenever this is completed, they separate themselves and have nothing more in common with them. Still others who are more intelligent [sunetotera] and shares in memory live a longer time and in a more political manner with their offspring.
one part of the life of animals, therefore, is devoted to the actions concerning reproduction, another part to the actions concerning food. For on these two all their serious pursuits and their life happen to concentrate. Their food differs mostly according to the matter of which they are constituted. For the growth of each will happen according to nature from this. Further, whatever is according to nature is pleasurable; and all animals according to nature pursue pleasure.
Notice the eight points that Aristotle makes in this passage.
1. Human psychic powers are more clearly differentiated forms of powers found in other animals. Thus, there is an unbroken continuity between all species of life.
2. These common powers include both traits of moral temperament (such as courage, for example) and cognitive capacities for reasoning and judging.
3. In comparing human beings and other animals, their powers are either the same or different in degree or similar by analogy. In the case of analogy, different powers serve similar functions.
4. These comparisons are clearest if one considers human infants, because in some sense there is no difference between a young child and a non-human animal.
5. Animals are naturally adapted for survival and reproduction, and therefore the practical structure of their lives varies according to how each species feeds and propagates.
6. Unlike plants, some animals must care for their young, and how they do this shapes the life of each species. Some animals only feed the young at the beginning of life. But those who are more intelligent develop more lasting bonds with their young. Thus, higher levels of animal intelligence are connected to more extended parental care.
7. Such long-term nurturance of the young is the natural ground from which political life grows.
8. Animals, having some capacity for sensation, act to satisfy their natural desires because by nature this gives them pleasure.
All of these points enter into Aristotle's moral and political science. Consider, for example, the importance of pleasure.
"Whatever is according to nature is pleasurable, and all animals according to nature pursue pleasure." Having developed this thought in his biological works, Aristotle incorporates it into the Nicomachean Ethics. All human action is governed by pleasure and pain, as is true for all animals (1104b4-5a16). A good man is one who feels pleasure and pain in the right manner. To deny the goodness of pleasure would be an implausible denial of the testimony of all animals. Since all creatures, the mindless as well as those with some rational judgment, pursue pleasure as good, it must be good in some sense. A man could not endure even Plato's Idea of the Good if it were painful to him (1153b25-36, 1158a24-25, 1172b35-73a5).
While pleasure is not always the direct object of desire, pleasures accompanies the satisfaction of every desire, which supports the observation that the good is the desirable (1174b15-76a29).
Each animal has a species-specific pleasure corresponding to its species-specific function, for pleasure accompanies the activity fulfillment of whatever inclinations constitute the nature of a species. Therefore, "the pleasures of a horse, of a dog, and of a human being are different" (1176a6). But although the pleasures of each species tend to be the same, what human beings find pleasurable varies to some extent. In extreme cases, human beings can be impaired or ruined so that what is pleasurable for them is not truly pleasurable in the sense of perfecting human nature. Such people don't understand what they truly desire (1153b25-36, 1176a3-22).
The pleasures of the good and the bad differ just as the pleasures of children and adults differ, and we can properly judge the pleasures of the good person to be superior just as we do those of the adult (1174a1-3, 1176b23-25). Some pleasures are natural to human beings (or to other species), while others are unnatural. Of course, the unnatural pleasures are natural in the weak sense that they have natural causes; but they are unnatural in the strong sense that they disrupt the normal balance or order in an animal's life. Among human beings, unnatural pleasures arise either through injury or through habit or through evil natures. So, for example, a person might be driven by a mental disease to kill and eat his mother. Or a person abused from childhood might take pleasure in sexual perversion. Or a person born with some mental disorder might take perverse pleasure in eating human flesh or abusing his children (1148b15-49a20).
Those who would guide the education of the young and the general formation of human character must understand the motivational power of pleasure and pain (1172a19-26). Consequently, "theorizing about pleasure and pain belongs to the political philosopher; for he is the architect of the end, to which we look in calling one thing good and another bad in an absolute sense" (1152b1-8). Furthermore, as a philosopher, he understands that the intellectual life is the most pleasurable insofar as a human being in the highest sense is his intellect (1178a5-8).
Darwin also concluded that human beings found their natural pleasures in satisfying their instincts for conjugal, parental, and social bonding generally. As refined by habit and reason, these instinctive pleasures shape the moral sense of human beings. Any person without such instincts would be an "unnatural monster" (2004, 136, 140, 148; 1987, 619-22). Moreover, Darwin thought that in some savage circumstances, bad habits and insufficient reasoning would lead people to commit "unnatural crimes" (2004, 140-50). But with the progress of civilization a few people, he believed, would discover the intense pleasures of the intellect as the source of the greatest human happiness (1987, 548-49).
For both Aristotle and Darwin, therefore, morality rests on a natural union of instinctive feelings and rational judgments. They would agree neither with Hume's exaggerated claim that reason is the slave of the desires, nor with Kant's exaggerated claim that reason discovers the moral law in complete abstraction from natural human desires. For Aristotle and Darwin, reason complements the natural desires by arbitrating conflicts between them to conform to a coherent plan of life, in which each natural desire finds its fullest and most appropriate expression.
Although perhaps never fully attainable, the goal of rationally ordering all the natural desires into a coherent whole can at least be more or less approximated. Success will depend not only on the individual's character and thought, but also on his physical circumstances and on the way of life of his political community.