Reasonable thoughts on the use of parts in humans, animals and plants (1725) and Singular phenomenon of fruit-bearing apple recalled all the way from blossoming to physical reasons (1727)

Wolff's physical writings continue: a book on general physical processes and another book on the purposes of physical processes are followed by a book concentrating specifically on biological questions, Vernünfftige Gedancken von dem Gebrauche der Theile in Menschen, Thieren und Pflanzen.

As we have seen earlier, biology and especially botanic was a topic Wolff himself had empirically investigated. It is then no wonder that the book feels quite professional and up-to-date, even if Wolff cannot yet know e.g. how leaves actually nourish plant in their interaction with carbon dioxide: it is enough that he can describe leaves and their parts and understands that they have something to with the nourishment of plants. Wolff goes carefully through all parts of human body, beginning from different types of fibers – the smallest elements of living matter known at that time – and all organs made out of these elements, and if necessary, he compares parts of human body with parts of other animals. After humans and animals, similar treatment waits plants.

What is somewhat striking is Wolff's open attitude towards even the most taboo questions of human bodies, particularly sexuality. Description of sexual organs was frowned upon, because reading about genitalia was thought to incite people to perversities. Wolff, on the other hand, thinks that information about sexuality will help a person to fulfill sexual needs in a moral manner: you cannot do something properly, if you don't know the reasons for it. Interestingly, Wolff is aware of the role of clitoris in female sexuality and thinks it has been created for awakening in women the want of intercourse - a necessary precondition of having children. Wolff also suggests that sexual pleasure is especially meant to encourage women to want sexual intercourse: otherwise they might refrain from it, because they feared the burden of child birth.

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As it should be evident, Wolff is not satisfied with mere description of living beings, but is also concerned to find the reason why God created them in the first place. The answer is actually familiar already from Wolff's teleology: humans and other rational entities exist in order to witness the glory of creation, and other things, including living entities, are meant to serve rational beings and their needs.

Wolff adds to this general description of the purpose of animals and plants two interesting details. Firstly, while Wolff's official teleological account of the world is an example of what later was called external teleology (things have a purpose beyond themselves), he also notes that animal and plant species could also be regarded as having an internal purpose that does not require reference to things beyond that species. Wolff suggests that this internal purpose would be propagation of species: animals and plants exist to produce other plants and animals of the same type. Thus, from the viewpoint of a certain plant, it is contingent that we use its flowers for medicinal purposes, but on the other hand, it is relevant that the flower produces seeds.

Secondly, Wolff also adds an aesthetic layer to his teleology. That is, he suggests that God does not just create purposive animals and plants, but that God's creations are also beautiful or pleasing to senses. Wolff especially emphasizes the symmetricity of animals as an evidence of their beauty: it just looks better if I have two ears equidistant from the center of the face and not, for instance, one on left hand and other on stomach.

Philosophically most interesting part of Wolff's biology are the little details that shed some more light on the question of the interaction of soul and body. Wolff refrains from the central question, whether and how soul and body interact, on the pretext that this is more of a metaphysical than biological problem. Yet, he points out that the problem concerns actually the interaction between the soul and the brain, which then controls the rest of the body. The exact details of this control are still not known to Wolff. Wolff is aware of nerves, but he doesn't really know how they work, so he must rely on Cartesian idea that there are some mysterious animal spirits that help nerves turn volitional movements of brain into movements of muscles, although instead of spirits, Wolff prefers the more material concept of nerve juice.

Before ending this post, I shall just quickly note that Wolff was actually more involved with biological studies at this time, because in a few years he published an inaugural dissertation celebrating his new post at the university of Marburg, Phaenomenon singulare de malo pomifera absque floribus ad rationes physicas revocatum, which is based on the botanical research of Wolff and his follower Thümmig.

So much for Wolff's biology. In next post, I shall investigate a Wolffian we have already met a number of times, tackling now metaphysics.