Thomas Aquinas followed the philosophy of Aristotle. So, he believed that the universe is organized so that each thing in it has a purpose or goal. Reason helps humans discover and achieve their goals. Actions which are in accordance with our natural goals are good and those that interfere with these goals are bad. So, for example, since we are capable of reasoning we should develop our intellect and since all living things are inclined to preserve their own lives, committing suicide goes against nature and is wrong.
Aquinas beliefs also include a theological framework. His beliefs support the Roman Catholic Churchs positions on abortion and contraception. One can see this by looking at two of Aquinas most well-known principles.
The Principle of Totality: An individual may not dispose of his organs or destroy their capacity to function, except to the extent that this is necessary for the general well-being of the whole body. Destroying an organ or interfering with its capacity to function prevents the organ from achieving its natural purpose.
The Principle of Double Effect: Aquinas recognized that there are times when the action you think you ought to do will have good and bad effects. In effect, you have an ethical dilemma or conflict.
Under these circumstances, it is permissible to perform an action causing bad effects if you meet these four conditions:
1. The action itself is morally neutral or morally good.
2. The bad effect is not the means by which the good effect is achieved.
3. The motive must be the achievement of the good effect only.
4. The good effect is at least equivalent in importance to the bad effect.
Of course, these criteria require a means by which you determine whether something is morally good, neutral, or bad. Aquinas used the combination of his Roman Catholic faith and Aristotles teachings. Someone else might use the same method, but with a different ethical foundation and come up with different answers.