Underlying Aquinas' ethical notions are a number of key presuppositions that a humanist ethic would not necessarily include. For instance it begins with the notion that God has created the world with purpose and that the final end of all things is in God, or an after-life, this is clearly a contention for humanists, He also begins with the assumption that human nature is more good than evil. However this is not a necessary presupposition.
It may look at times that people are driven more by good but is this merely social conditioning? In fact a careful consideration of people may reveal them to be more selfish than not. Furthermore, is nature innately good as well? All too often we see unnecessary suffering and pain caused both by nature to humans and humans to each other. Postulating the innate goodness of humans and nature really depends on one's presuppositions and interpretation of the 'evidence'.
Aquinas' premise of the natural goodness of nature contradicts the traditional notion of human sinfulness and creation falling, and their need of spiritual regeneration by God, as traditionally believed, and taught, by a large majority of the Church. Apart from these contentions we also need to find reconciliation between Natural Law and the theory of evolution. If the cosmos is expanding outwards, or contracting inwards as some may postulate then the notion that everything has a fixed telos is brought into question. If the universe is ever-changing can there be fixed moral principles?
In the light of this one needs to consider also whether ethical principles lie beyond the material realm which is known as metaphysics or are expressed within the material realm, innate. Taking the humanist or theist religious contentions a stage further one can begin to see that the Natural Law ethic raises more problems, in this context, with the Christian believer's claim to access special moral truths from God. For instance, if moral principles can be reasoned by simple inner reflection then the Bible, as God's Word, looks merely to be a more revered expression of the Natural Law Theory.
In theory one could argue, as the Apostle Paul does in the letter to the Romans that people have access to God's law in their conscience but from here we must ask whether God's law, for example the Bible is subordinate to reason or is reason subordinate to God's law. If we say the former then, for example, what is so special about the Bible? If we say the latter then how is it possible for God to judge and condemn those who have acted according to their conscience rightly or wrongly, solely because they have not had access to, or understanding of, the Bible?
Furthermore, subordinating either faith above reason or reason above faith seems to depend more on one's initial presuppositions, epistemology than to any ontological priority in either notion. Advantages with Natural law are that it is universal and is an objective way of deciding right and wrong, as well as the fact that it offers general rules that can be applied to actual situations while avoiding the relativism of situation ethics. The Natural Law theory also avoids ambiguities in systems that involve assessing consequences, for example Utilitarianism.
Problems with the Natural Law Theory include that is not possible to move from an "is" to an "ought. " It is also very difficult to come to some kind of agreement on what it is to be human and it is difficult to be certain what everything's purpose actually is, for example is the purpose of sexual intercourse only to procreate, or could we include the strengthening of a relationship as well? Also some animals are naturally promiscuous, can we read from nature to include this into our behaviour?
Other problems with Natural Law are that it makes masturbation worse than rape, and some would argue that Natural Law does not work because it does not take into account the existence of God, and therefore there is no secular Natural Law. On the surface Natural Law works quite well with sexual ethics. We can read from nature that through the sexual act procreation occurs. Hence, it is part of the course of nature that the end of sexuality is procreation. This then means that any form of sexual activity is immoral.
But is there more than one purpose to the sexual acts. The Catholic Church says that a sexual act must be open to procreation, but has more recently emphasised the deepening of the relationship between couples. What if there was a gene for homosexuality, then a person's genetic setup would incline someone to a different goal. Charles Curran takes on the Catholic Churches notion of Natural Law. Curran thought that it was too restrictive, and that it put too much emphasis on the physical approach.
Curran wanted to see Natural Law take the whole of the human person into consideration; this would include modern notions of psychology and genetics. Proportionalism has always been a part of the Natural Law Theory. Its fundamental position is that you can use moral law to an extent to work out right and wrong, but whereas Natural Law Theory believes that there are intrinsic evils, proportionalism maintains that there might be Proportionate reasons where one might have to against one's basic morality.
Bernard Hoose is the most notable British exponent of Proportionalism, he believes that Natural Law on the whole is correct, but on occasions there maybe Proportionate reasons to go against traditional Natural Law. The "evil" caused as a result of aiming at a proportionate good, such as the scars or loss of a limb, are called ontic evils. Therefore there are no intrinsic evils, whereas the Catholic Church believes in intrinsic evils. A problem with proportionalism is how do we judge whether an action is proportionately good? That is to say, how do I know that I should use surgery even though I'm inflicting pain.