Natural Law Theory (NLT) is an ethical theory derived from the thinking of people such as Thomas Aquinas that attempts to establish that humans, for example, have an ideal form or essence that dictates how they should act. The form of a particular species of bird is that it has feathers, a beak, two eyes, can fly, has a particular colouration and so on. The essence of a bird can be described by listing, one assumes, its properties. There is, in reality (so they would say), some objective notion of what these properties are.
For all of these thinkers, literally everything has this kind of essence, though those essences will differ between things. The idea that homosexual humans (I use this as an example, many other properties could also be used) are morally wrong is derived from the notion that they have an essence, a natural form, to which they should adhere, but do not. A good badger is a badger that most resembles the essence of a badger. A good human is a human who most resembles the nature or essence of a human. Homosexuals or some other group of supposedly morally bad people are morally bad because homosexuality is not a property of the human essence, or essential property.
To confuse matters, we could subcategorise humans in terms of male and female as well. In fact, one of the problems with essentialism and Thomistic philosophy is that you could subcategorise anything further and further to create more and more essences until you eventually have an individual instantiation of a thing. For example, you could subcategorise humans into males and females. But why not continue with other categories? Age, hair colour, size, geographical distribution, skin colour and so on but each of these categories could be sliced and diced even further. Who gets to define the categories? Of course, such advocates of NLT or Thomism would say that God gets to define this, but how do we know what those categories are? We can look around us at the natural world, but as I have at length set out before, categorising the natural world in light of evolution is utterly problematic.
In terms of categorisation, it appears to me that Thomists are not bothered about hair colour or skin colour (although I am sure many of them could be!) and they are happy to allow this variation in light of an overarching human essence. However, they are employing a double standard. They seem to allow variation in physical characteristics but not in mental characteristics. Let’s take homosexuality. Someone who has homosexual tendencies is seen as morally bad because they are not adhering to the essential form of a human. But homosexuality isn’t just a decision that people make. There is a huge amount of research to show that there are very real material bases for homosexuality, particularly in men. Whether it be genetic or biological in nature, there is a clear parallel with other genetic and biological differences between humans that can apparently show a massive variety of physical difference and this does not invalidate them as being a “good” human. However, as soon as they diverged from a very strict mental or rational blueprint, they are deemed as “bad”.
To look at this further, then, we have a set of mental properties that fulfil the criteria of the essence of a human. Again, we have the problem of defining and categorising exactly what these are. Because this appears not to be the case of employing moral reasoning to define moral goodness, but adhering to the essential form of a human. We could have a circular argument where the essential form of a human includes moral behaviour and properties that are good, and these are themselves defined as behaviours that are essentially human. And round and round we go. This is a dilemma paralleled in the Euthyphro Dilemma.
One of the properties of the essence of a human is the property of rational thought. But defining what this is ain’t easy, and falls into the same traps as any property that sits along a continuum as can be seen in the image above. There are humans that have different levels of rationality and at different times, and there would have been an evolution of this over our biological history. Was there a single point in time where two “non-rational” hominids gave birth to a “rational” hominid that finally qualified as “human” in terms of its essence or form?
NLT supposedly allows Christians to claim homosexuality or homosexuals are morally bad because there is a prescriptive human essence from which such urges or biology or behaviour diverges. Homosexuality is not the “function” of humanity, so to speak (what Aristotle called the final cause). Of course, we have the whole debate around libertarian free will: if it can be argued that an individual doesn’t have the power of volition over these urges or biology or behaviour, then what does this say about such a moral framework as NLT?
The whole project appears doomed to confusion and arbitrary delineation.
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