“Now it is evident that the higher the principle the disorder of which causes the disorder in the humors, the graver is the sickness: thus a sickness which comes on the human body from the heart, which is the principle of life, or from some neighboring part, is more dangerous. Wherefore a sin must needs be so much the graver, as the disorder occurs in a principle which is higher in the order of reason. Now in matters of action the reason directs all things in view of the end: wherefore the higher the end which attaches to sins in human acts, the graver the sin.
Now the object of an act is its end, as stated above (Q. 72, A. 3, ad 2); and consequently the difference of gravity in sins depends on their objects. Thus it is clear that external things are directed to man as their end, while man is further directed to God as his end.”