I. The natural law gives no rights except such as are finally founded in human nature itself; but human nature cannot give a title to dominion over one's own life; therefore the natural law does not give man such a right.
Every natural right is either congenital or acquired. The title to a congenital right is human nature itself; the title to an acquired right is some act consequent to the exercise of human activity. The right to such exercise is, in turn, congenital and founded in human nature.
If nature established the title to dominion over one's own life it would thereby establish the power of destroying that life, and thus of removing the fundamental title to all rights; but nature exists as the foundation for rights, not for the subversion of rights; therefore human nature cannot give a final title to dominion over our own life.
Again, this minor of the first argument is confirmed by the fact that if nature even remotely established the power of self-destruction there should be in nature itself some natural tendency to such destruction, but the direct contrary is the fact.
II. The natural law cannot grant a right to man which is not a means to the common end of human life; but absolute dominion over one's own life is not such a means, therefore the natural law cannot give one dominion over his own life.
The natural law is only an ordination of man to that common end of human life and to the means toward that end. As regards the minor of this second argument, an absolute dominion over his own life would give man power to stop all his human activity, yet the common end of human life is attainable only by man's activity. The stopping, or the power of stopping, all activity cannot be a means to that end.
III. The natural law cannot give man a power which is opposed to the essential needs of human nature itself; but that a man should have absolute dominion over his own life is opposed to an essential need of human nature itself, therefore the natural law cannot give such a power.
Dominion over his own life implies the power in man of rebelling against the subjection which he owes to God; but human nature essentially demands that man be in subjection to God, since dominion over one's own life and subjection to God are contradictory.