The author notes that for Aquinas, while synderesis is the habit of knowing the precepts of natural law, conscience is the active application of such precepts in particular circumstances.
For Aquinas, says the author, without the application of the moral virtue of prudence, there is always the danger of developing an erroneous conscience. While Aquinas does say that we have an obligation to follow our conscience, even an erroneous one, we also have an obligation to form our conscience in accordance with right reason.
The section on Aquinas on natural law is very helpful. For Aquinas, natural law is nothing other than the participation in eternal law by rational creatures. The first principle of natural law, says Aquinas, is that good is to be done and evil avoided. It is our natural inclinations that ground our understanding of natural law principles. Actual belief in God is not required to pursue natural law ethics. So this version of ethics really does belong in text books on secular ethics.
Aquinas divides the virtues into intellectual and moral ones. Prudence is an intellectual virtue since it is required for one to will a morally good action. The virtues are thus closely related to natural law since living in accordance with natural law leads to a growth in the virtues. One's moral dispositions - that is the virtues and vices one cultivates - regulates the relations of one's passions to the dictates of practical reason. The author gives the example of two married individuals who are drawn towards an adulterous union with each other. One individual has cultivated the virtue of fidelity while the other has not. The virtues and vices direct the individuals towards the choices they make. Having made one's choice, the intellect commands the will to use one's rational capacities to employ the choice means.
A reading of this book enables us to see how closely woven the topics of conscience, natural law and the virtues are for Aquinas. The author returns briefly to natural law theory at the end of the book when he examines how natural rights and natural law are related.
What we have here is a really helpful introduction to a great work of theology.