I answer that, By "natural" we mean that which is caused by nature, as stated in Phys. ii, 1. Consequently the question as to whether a particular passion is more or less natural cannot be decided without reference to the cause of that passion. Now the cause of a passion, as stated above (Question [36], Article [2]), may be considered in two ways: first, on the part of the object; secondly, on the part of the subject. If then we consider the cause of anger and of desire, on the part of the object, thus desire, especially of pleasures of the table, and of sexual pleasures, is more natural than anger; in so far as these pleasures are more natural to man than vengeance.
If, however, we consider the cause of anger on the part of the subject, thus anger, in a manner, is more natural; and, in a manner, desire is more natural. Because the nature of an individual man may be considered either as to the generic, or as to the specific nature, or again as to the particular temperament of the individual. If then we consider the generic nature, i.e. the nature of this man considered as an animal; thus desire is more natural than anger; because it is from this very generic nature that man is inclined to desire those things which tend to preserve in him the life both of the species and of the individual. If, however, we consider the specific nature, i.e. the nature of this man as a rational being; then anger is more natural to man than desire, in so far as anger follows reason more than desire does. Wherefore the Philosopher says (Ethic. iv, 5) that "revenge" which pertains to anger "is more natural to man than meekness": for it is natural to everything to rise up against things contrary and hurtful. And if we consider the nature of the individual, in respect of his particular temperament, thus anger is more natural than desire; for the reason that anger is prone to ensue from the natural tendency to anger, more than desire, or any other passion, is to ensue from a natural tendency to desire, which tendencies result from a man's individual temperament. Because disposition to anger is due to a bilious temperament; and of all the humors, the bile moves quickest; for it is like fire. Consequently he that is temperamentally disposed to anger is sooner incensed with anger, than he that is temperamentally disposed to desire, is inflamed with desire: and for this reason the Philosopher says (Ethic. vii, 6) that a disposition to anger is more liable to be transmitted from parent to child, than a disposition to desire.