Philosophy as Biology
In the 1960s linguistics took a biological turn with the work of Lenneberg and Chomsky.  Language was held to be genetically fixed, a species universal, just like the anatomy of the body. It is a biological aspect of human beings, not something cultural or learned, more like digestion than chess. Language evolved, became encoded in the genes, and is present in the brain at birth. Since linguistics is properly viewed as a branch of psychology, according to these theorists, this means that part of psychology is also biological, not something separate from and additional to biology. But then it is reasonable to ask whether more of psychology might fall under biological categories; and succeeding years saw psychology as a whole taking a biological turn. Many of our mental faculties turn out to have biological origins and forms of realization in the organism. Indeed, learning itself must be genetically based and qualifies as a biological phenomenon: what an animal learns is part of its biological nature, not something set apart from biology. True, what is learned is not innate, but many things are not innate that are part of the natural life of the organism (e.g. a bee’s knowledge of the whereabouts of nectar). Dying by predation is not innate but it is certainly biological. Biology is the science of living things, and living things learn as part of their natural way of life. In any case, psychology turned from cultural conditioning to biological naturalism; it became evolutionary. How could it not given that minds evolved along with bodies? The mind of an organism is part of its nature as a living thing; it doesn’t exist outside the sphere of biology (as the soul was supposed to). The organism is a psychophysical package.
The basic architecture of language is thus a biological architecture. Syntax is an organic structure; the lexicon is a biological system too. When we study these things we are studying the properties of an organism, just like its other biological properties. They had an evolutionary origin in mutation and natural selection, and they have a biological function (probably to enhance thought, as well as serve in communication). One of the organs of the body, the brain, serves as the organic basis for language, as the heart serves as the basis for blood circulation. So linguistics (descriptive grammar) is not discontinuous with biology but part of biology.  It had conceived itself differently, perhaps out of a feeling that language raises us above the level of the beasts, but in these post-Darwinian times it should be relegated to biological science. Freud had made similar moves in affective psychology; the biological school in linguistics was moving in the same general direction. This broke down the old dualism and established the study of language as a department of biology, even when it came to the fine structure of grammar.
This is an oft-told tale (though still not without its detractors), but it has not yet colonized the entire intellectual landscape. Recently there has been a movement to classify consciousness as a biological phenomenon: it too is innately determined and biologically functional. Organisms have consciousness the way they have blood and bile—as a result of biological evolution and bodily mechanisms. It is not something supernatural, an immaterial infusion. That certainly seems of a piece with the biological naturalism that has dominated psychology in recent decades, but does it go far enough? Can’t we also announce that phenomenology is a branch of biology? That is, the systematic phenomenology of Husserl is really a form of biology: the very structures of consciousness are biological facts. Husserl doesn’t suspend the natural sciences (the epoche); he promotes one of them. Phenomenology is the study of a biological aspect of the human mind (and bats have their phenomenology too), just as linguistics is the study of a biological aspect of the human mind (and bees have their language too). When Sartre characterizes consciousness as nothingness and explores its modalities he is doing biology, because consciousness is a biological phenomenon—evolved, innately programmed, functional, and rooted in tissues of the body. To be sure, it is not reducible to other biological facts (such as brain structures); it is a biological fact in its own right. But it is a biological fact nevertheless—part of the life of a living thing. Its essence is nothingness, as the essence of the heart is pumping and the essence of the kidneys is filtering. It has a certain natural architecture, established by the genes, in both humans and animals. We certainly don’t choose its essence. In so far as consciousness exhibits universals (intentionality, qualia, transparency), those are biological universals, like the universals of human grammar. Phenomenology thus belongs with psychology as a branch of biology. Biology deals with living things–as opposed to physics, which deals with non-living things—and the mind is an aspect of life. Husserl could have cited Darwin (correctly understood): The Origin of Species of Consciousness. This is not biological reductionism, simply the acknowledgment that biology extends beyond the body. It is not that religion takes up where biology leaves off.
I take it I am not shocking the reader unduly. Isn’t this all part of our current secular scientific worldview? Biology by definition encompasses the life sciences, and linguistics, psychology, and phenomenology are all parts of the life sciences. Speaking, thinking, and experiencing are all modes of living—what living things do (some of them). They are, as Wittgenstein would say, aspects of our “form of life”, part of our “natural history”. Maybe we need to expand our conception of biology beyond the typical curriculum, but it is not difficult to see that these aspects of our nature properly belong to biology, broadly conceived (certainly not to the physical sciences). However, I now wish to assert something that may strike readers as pushing it just a bit too far: philosophy too is a branch of biology. I don’t say this because I think philosophical questions reduce to biological questions; I say it because of the methodology of philosophy. We hear about the “linguistic turn” in philosophy—using the study of language as a means of arriving at philosophical conclusions about ground-floor questions. But given the biological turn in linguistics this implies that philosophy has already turned into a branch of biology. Language is a biological phenomenon and it is held to be the foundation of philosophy, so philosophy is based on a sub-discipline of biology. If the logical form of sentences is deemed central to philosophy, then it is the form of a biological entity that is in question. Logical form, like syntax, is an aspect of an evolved and biologically based entity—the architecture of a biological trait of humans. If speech acts are deemed central, then this aspect of living things will assume methodological importance—as opposed to acts of reproduction or respiration or excretion. The combinatorial power of language has rightly received considerable attention, but this too is an evolved biological trait. The biological turn in linguistics combined with the linguistic turn in philosophy together imply the biological turn in philosophy.
But what if we reject the linguistic turn? What was it a turn from? Mainly it was a turn from a more direct investigation of concepts. But investigating concepts is also investigating a biological phenomenon. Let me put it bluntly: a concept is a living thing. A concept is like a cell of the mind (and note that biological cells were so called because of their resemblance to the living quarters occupied by monks). Concepts are the units that make up thoughts and other mental states, as words make up sentences. Concepts have functions, they evolved, and they are rooted in organic structures of the brain. So when we study concepts philosophically we are studying entities as biological as blood cells or enzymes. We scrutinize these things for their philosophical yield, not for their contributions to biology as such, but they are still biological entities. To be sure, we are interested in their content not their physiology, but having content is just another biologically fixed fact about them. Even if you think concepts are acquired by abstraction, they are still entities that exist in the context of a living organism (like big muscles or manicured nails). Conceptual analysis is the dissection of a biological entity; it is not the examination of a disembodied abstract form. There might be such forms, but they must be reflected in the natural traits of organisms at some level. We have no trouble recognizing that an animal’s concepts are biological forms; human concepts are not different in kind. Bee philosophers can reflect on their bee concepts (or turn their attention to bee language), and human philosophers are in the same case—reflecting on their biologically given traits.  How they do that must also be rooted in biology, but the important point is that thinking is a biological fact; and in so far as philosophy concerns itself with “the structure of thought” it is a biological enterprise. The results don’t concernbiological matters, as opposed to matters in the world at large, but the method involves surveying a certain class of biological entities. Analyzing a concept is analyzing a living thing—as much a living thing as any organ of the body. Our intellectual faculties are indisputably aspects of our life as organic beings, and concepts are just their basic components—as cells are the basic components of bodily organs. It follows that philosophy is (a branch of) biology. Philosophy could be called conceptual biology.
I want to emphasize how biological concepts are. First, they arise through the evolutionary process (though we have little understanding of how this happened). Second, they are manufactured during embryonic development as a result of genetic realization (or if you think they are acquired later, it is by biological means, e.g. abstraction). Third, they have a biological function—to enable thought, which enables rational action. Fourth and crucially, they must be realized in some neural mechanism that enables them to have their characteristic features, chief among which is their combinatorial powers. The neurons must be able to hook up with other neurons so as to produce complex thoughts; and this hooking up must respect the logical relations inherent in thought (it’s not just a matter of brute aggregation). There must be a physiology of thinking, and specific to thinking. So concepts cannot somehow float above the biological substructure; they depend upon it. Presumably this implies some sort of hidden structure to concepts analogous to the hidden structure of the cell (nucleus, mitochondria, etc.) Concepts are biological through and through. So if they are what philosophy investigates philosophy is up to its ears in biology. It would be different if philosophy could pursue its interests without recourse to concepts, say by simply looking at the extra-conceptual world, but that idea is hopelessly wide of the mark. And even if you think that someparts of philosophy require no reference to concepts, much of it clearly does (the parts that expressly analyze concepts, in particular). Philosophy is thus one of the life sciences and should be understood as such. There are the sciences of the inorganic world—physics, chemistry, astronomy, geology—and there are the sciences of the organic world—zoology, biology, genetics, biochemistry: and within this broad grouping linguistics, psychology, phenomenology, and philosophy fall into the latter category. As I say, this is no form of biological reductionism or determinism, simply a taxonomic observation. It is making explicit what has been implicit since the time of Darwin. 
I want to end with a point about mathematics. The kinship between mathematics and philosophy has long been recognized; in particular, the status of mathematics as a non-empirical conceptual inquiry makes it similar to philosophy. So is mathematics also a department of biology? Well, if we view it as investigating the implications of basic mathematical concepts it presumably is, for the same reasons philosophy is. Mathematical concepts are products of evolution too, and they must have an underlying physiology. They too are living things. To the extent that mathematical concepts are part of the subject matter or method of mathematics, that subject is also fundamentally biological. Suppose mathematical ideas are innate, just as the classical rationalists supposed; then they must have evolved by mutation and natural selection, become genetically encoded, and matured in the individual organism’s brain to become the conscious entities we now know. Investigating these concepts is thus an exercise in biological exploration—discovering what these evolved traits have hidden in them. How they evolved we don’t know, but if they did evolve then mathematics is another kind of life science, mathematics being part of human life. The concept of number, say, is part of our evolved form of life (quite literally). Counting is like speaking—a human universal. Mathematical theory is the spelling out of the mathematical concepts we inherited from out ancestors.
 The work of Ruth Millikan is also an instance of the biological turn in linguistics and psychology, to be set beside Lenneberg and Chomsky. The biological concept she emphasizes is function as distinct from innateness.
 No one would doubt that the study of bee language belongs to biology (zoology to be precise), but it took some persuasion to get people to accept that human language is part of human biology (zoology). If bees had philosophers it would be clear enough that these philosophers are studying a biological phenomenon—bee language or bee thought. Is it that there is resistance to the very idea of human biology?
 In retrospect we can see the work of Locke and Hume (among others) as a form of human biology: they undertook a naturalistic study of the human mind, turning away from scholastic essences and the like. If they had known about Darwin, they might have welcomed the biological naturalism inherent in his work.