The latest book from the philosopher Daniel Dennett is a work of tremendous ambition. Its scope is displayed in its title: Dennett spins a story of the evolution of intelligence that begins with unicellular life and culminates in human genius and the capacity of the human mind to comprehend its own origins. Like all of Dennett’s writings, From Bacteria to Bach and Back is brimming with erudition, wit, and insight. He has always been one of the rare philosophers who can write in a way that is at once philosophically rigorous and accessible to an educated and interested non-philosopher. This new book is no exception.

Nor is it an exception to something that many readers will be less enthusiastic about—namely, Dennett’s unapologetically reductive understanding of life, consciousness, intelligence, and culture. This reductionism has two main aspects. First, Dennett holds that the behavior of complex systems, including living ones, is generated entirely from the interactions between their parts. Second, except in the special case where humans or other animals act as the “intelligent designers” of our tools or cultural artifacts, he also describes the evolution of complexity and intelligence in an entirely bottom-up way—as the result of blind selection pressures acting on simpler and less intelligent systems. Dennett explains the latter idea with a pair of images that are familiar from his other books: instead of “skyhooks”—mythical devices that hang from the sky in order to move things to new heights—nature makes use of cranes, which lift things from the ground up. Reductive explanations of the first sort mean that we don’t need souls or other distinctively higher-order principles to explain what happens in nature. Explanations of the second sort mean that no appeal to God or other supernatural powers is required to explain why things are the way they are.

Things are not quite so simple as that. For as Dennett acknowledges, there is more than one kind of explanation. Suppose, for example, that I ask you for an explanation of Donald Trump’s election to the presidency. One way to answer this request would be with an account of the causal factors that contributed to Trump’s victory. Such an account might discuss the role of the Electoral College in granting disproportionate power to rural states, the deteriorating economic situation of white working-class men, the effects of immigration, the influence of the conservative media, and the ways that misogyny and xenophobia may have made Trump and his policies popular. This account addresses the question “Why did so many voters choose Trump?”—where “Why?” is understood to mean “How come?” It is an attempt to explain what brought it about that so many Americans (enough, though not a majority) voted for Trump.

A second way to explain the same outcome would be in terms of the goals of Trump’s supporters, or their purpose in settling on him as their preferred candidate. There will likely be some contact between this sort of explanation and the previous one: Trump supporters wanted to restrict immigration, to improve the situation of white working-class men, and so on. But now these factors are treated not just as forces that made it likely Trump would win—as, for example, the structure of the Electoral College made this likely by giving more influence to the average Trump supporter than to his average opponent. As Dennett puts it, instead of answering the question “How come?” an explanation in terms of goal or purpose speaks to our desire to know “What for?” And the possibility of giving a how-come explanation of something in terms of impersonal, bottom-up forces doesn’t mean that the same thing can’t also be explained in terms of purposes or goals.

To illustrate this point, consider how we understand the growth of a tree. To explain how come a tree grows in a certain way, we might need to appeal only to factors like the genetic information in its cells, the nutrients in the soil, the chemical reactions involved in processes like photosynthesis, and so on. But the possibility of giving such a mechanistic, bottom-up explanation of the tree’s growth doesn’t mean that we can’t also appeal to goals or purposes in explaining what certain aspects of the tree’s metabolism or anatomy are for. The purpose of growing leaves, for example, is to help absorb energy from sunlight; the purpose of roots is to collect water from the ground; and so on. Just as we understand Trump’s election better when we appreciate the goals that motivated his supporters, this purposive or “teleological” explanation (from telos, the Greek word for “end” or “purpose”) of the tree’s growth captures something important that is not conveyed by a description of the cellular and genetic mechanisms that underlie this process.

For some scientists and philosophers who try to be especially hardheaded in their reductionism, talk of the purpose of roots or leaves is either foolish anthropomorphism (how can a mindless organism have goals of its own?) or involves a covert appeal to an intelligent designer whose purposes these are (e.g., perhaps God made trees have leaves because he wanted them to have a way to absorb energy from sunlight). To his credit, Dennett does not hold that teleological, what-for explanations of natural phenomena are all illusory or second-rate. He rejects the assumption that even mindless nature is necessarily bereft of purpose. There are, he says, purposes or “free-floating rationales” throughout the living world, and explanation in the biological sciences makes constant appeal to this natural teleology.

For Dennett, what makes the rationale of a process like photosynthesis or a structure like the roots of a tree free-floating is that this rationale does not need to be grasped or represented by anyone in order for it to be part of what explains a given phenomenon. In this respect, teleological explanation in biology is different from the explanation of human behaviors and artifacts in terms of our purposes or goals. Why did Jane go to the store? Suppose it was to get some milk: this explanation presumes that Jane believed that there was milk at the store, and that going to the store was a good way to get it. Why do hammers have flat heads? Because this is part of a clever design—our design—for driving in nails. But why do the roots of a tree grow outward? If it is in order to bring in more water, this need not be because the tree understands that an extensive root system will have this effect, nor because someone designed trees to be effective in finding nutrition. Rather, it just happens that roots are an effective way of absorbing water from the soil, thereby increasing the lifespan and improving the reproductive fitness of certain species of plants. For this reason, a process of natural selection that favors reproductive fitness was likely to lead to the spread of organisms that possess this favorable trait. This kind of rationale can exist whether or not there are beings like us with the capacity to comprehend it. The rationale for roots existed at least as soon as roots did—and long before human beings discovered what it was.