On one level, no there can be no conflict between natural and divine law. However, there may be conflict between the two in how they are embodied on an individual or cultural level. There may also be the appearance of conflict if we don’t properly distinguish the principles from the proper conclusions.
For an excellent introductory guide to Aquinas follow this link.
Human beings, in having a natural inclination to “the fitting act and end”– which is ultimately “eternal reason” — exhibit in our rational and creaturely existence, a participatory relationship between natural law and divine law (Thomas Aquinas: Selected Writings, Penguin Books., London 1998, p. 620). The rational recognition and obedience to natural law is the means by which we participate in the divine law (p. 620). Our rational natures, our desire and ability to contemplate about things beyond the merely material, and to concern ourselves with things not merely limited to material well-being, should provide us with a hint that the “fitting act and end” exceeds our material or natural capacities. For this reason, God gives us divine law. Thomas gives four reasons for God revealing to us his divine law. First, as our ultimate end exceeds our natural capacity in being directed towards “eternal happiness” we need divine direction in order to attain our ultimate end. Second, we are not accurately able to judge on “contingent and particular things” and so we require certitude of judgement which can only come from God. Thirdly, as our “fitting act and end” involves our interior intellectual lives, natural law is not adequate to govern or “restrain interior acts.” Fourth, in order for there to be final clarity and justice in the mixture of practical and particular judgements, all things will ultimately be judged according to divine law (p. 623).
In none of these things is there a conflict between natural and divine law. The one serves as a floor upon which we may, by the light of faith, break through the ceiling of our materiality and apprehend and participate in divine law. And as God is simple/holy and infinitely Good, it is not possible for the principles originating in him and sustained by him to be in conflict. “God through his wisdom is the maker of the universe of things… the notion of divine wisdom moving all things to their fitting end takes on the note of law… the eternal law is nothing other than the idea of divine wisdom insofar as it is directive of all acts and movements” (p. 633).
However, in terms of how human beings exhibit this participatory relationship between natural law and divine law, there is conflict. While human affairs are subject ultimately to eternal law, in both our knowledge and our actions we are in varying degrees “imperfect” and “corrupt” (p. 640). Insofar as our knowledge of eternal law and natural knowledge are obscured by “passions and the habit of sin” a holy/perfect participatory relationship between natural law and divine law is “defective” (p. 640-641). The conflict is in proportion to how “bad” one is. On the other hand, “the good are perfectly subject to the eternal law, as always fulfilling it” (p 641). Also, there may be failure in this regard in terms of “rectitude” (there may be an impediment in nature (e.g., are psychopaths born that way?) and knowledge (if one’s reason has been “depraved” by bad customs, or a “bad cast of nature”). This is to say that there may be a general culture which depraves or malforms us in relation to specific moral principles (p. 649). Finally, a conflict may arise as a consequence of prolonged damage to the human heart. This may be a result of “bad persuasion” regarding speculative matters, depraved customs, or corrupt habits. Such things may result in the natural law being “erased from the hearts of men” (p. 652).
Finally, there may be the appearance of conflict if we don’t distinguish between the common principles and the proper conclusions (i.e., secondary precepts or particular applications). Whereas there is “the same truth or rectitude for all “in reference to “common principles of reason” (e.g., “one cannot simultaneously affirm and deny something” p. 644) this does not mean that everyone knows matters pertaining to “proper conclusions” of speculative reason to the same degree (e.g., the outworking of speculative reason resulting in specific of geometrical definitions not known to all). Neither do the common principles work themselves out in a uniformed way, as the “proper conclusions” of practical reason will vary depending upon the specific circumstances (p. 648). Also, relating to the reality of circumstantial and cultural change, whereas the “common principles of reason” are immutable, the natural law may change by way of addition or subtraction in reference to particular contexts or situations (p. 650). This addition or subtraction will, if proper and good, leave the common principles uncorrupted, but effect the proper conclusions in a generally applicable way for a particular culture. For example, the principle of “everyone should act according to reason” results in the proper conclusion regarding terms of borrowing goods and returning them to the rightful owner. However, there may be culturally or regionally unique circumstances that may make the terms of borrowing (and the rectitude of returning them) different (p. 648). For example, in peacetime, a scythe has simpler associations than during a time of war. It may be right not to return such a tool during wartime if it threatens the safety of the owner or others. If we don’t keep the difference between the common principles and the proper conclusions clear, we may think there is a conflict between natural law and divine law, when in fact there is only a difference in the outworking of natural and divine law in a local or individual context.
Mind your Maker 