Josef Fuchs is a Jesuit priest who has written on natural law not only for Roman Catholics but in order to foster a broader understanding between Catholic and Protestant moralists. In the following excerpt he describes in general terms a number of works in the natural law tradition. In these the being, the very essence or nature of man as composed of body and spirit appears as a norm of moral behavior and of law. Numerous paragraphs in the texts show this clearly. Marriage is 'by its nature' monogamous.
Reason reads the natural law in the nature of all things and particularly in the nature of man.
To say that reason is able to read the law written in the heart of man means simply that reason is able to grasp the law of nature from the ontological reality of man and of all things.