Natural Law C/B grade summary notes


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Aquinas’ Natural law ethics AO1

  • Telos – our purpose is to glorify God by following the primary precepts of natural law
  • Aquinas thinks we are all born with an ability called synderesis – the ability to know first the synderesis rule – to do good and avoid evil – and then the primary precepts of natural law (preserve human life, reproduce, educate, orderly society, worship God).
  • We then apply the primary precepts to moral actions/situations and get a secondary precept – a judgement on that particular action/situation. This process is called conscientia.
  • E.g. euthanasia goes against the primary precept to preserve human life – therefore ‘euthanasia is wrong’ would be a secondary precept.
  • Double effect. Some actions have two effects – one that goes against the primary precepts, and one that fits with them. 
  • E.g. killing someone in self-defence – one effect is killing someone, but the other effect is saving your life.
  • Aquinas says in situations like this, it is morally acceptable so long as you intended to bring about the good effect and the bad effect was beside your intention.

Modern science’s rejection of final causation (telos)

  • There’s no scientific evidence for purpose/telos – science can explain everything in the universe, or is at least progressing towards explaining everything, without needing the concept of ‘purpose/telos’. 
  • It looks like an outdated unscientific term that people just project onto reality.
  • Purpose only exists in people’s minds.


  • Purpose is nonetheless an important part of human experience and life – science can’t explain everything, in fact there’s so much about humans and the human mind that it is very far from explaining. It also can’t explain why the universe began – so it can’t explain why we are here, so there actually could be a purpose to human life. 
  • Science cannot rule that out.

Fletcher’s critique of Natural law ethics

  • Cross-cultural moral differences – different cultures have different moral views. –
  • This suggests Aquinas is wrong to think we are all born with the same moral compass.
  • In Medieval Europe there was much less awareness of other cultures compared to today.
  • If were all born with synderesis – then we’d all know the primary precepts and we’d have the same moral views – not different 
  • So human nature does not have the telos of following the primary precepts.

Defence of Aquinas: 

  • Cross-cultural core similarities – however, 
  • Although there are differences between cultures  – there is also a core set of moral views all cultures share – and they are very similar to the primary precepts!
  • All cultures value reproduction and education – no society allows killing for no reason – same goes for stealing (orderly society).
  • So, it looks like the primary precepts are universal and so they are part of human nature and we do have a telos to follow them.

Aquinas’ Natural theology vs Karl Barth’s protestant critique

  • Human reason is too corrupted by original sin to trust its ability to know the primary precepts or to apply them correctly. 
  • Barth argued our finite human minds have zero ability to know anything about God’s infinite nature, including God’s morality.
  • Christians should only get their morality from faith in the Bible.

Defence of Aquinas

  • Aquinas argues that the soul was not totally corrupted by original sin – otherwise we would just be on the level of animals, which we clearly are not.
  • So there must still be a part of us that is orientated through telos towards the good and allows us to know the primary precepts.
  • Aquinas would admit we can still be corrupted in our application of them, but it’s still overall beneficial to follow natural law ethics.

Whether the double effect is unbiblical 

  • Think about the Biblical moral law – e.g. ‘do not kill’ in the 10 commandments. 
  • There’s no fine print about killing being ok if it’s in self-defence and beside the intention.
  • So, Aquinas’ theory of the double effect clearly goes against the Bible!


  • Natural law different to the Bible – 
  • Aquinas accepts the divine law (the Bible) – but claims it is different to the natural law. 
  • God gave us the Bible – but he also gave us the natural law in our nature and wants us to follow that too. 
  • It can’t be unbiblical to do what God wants.