It is impossible to recognise what jealousy is without a knowledge of what zeal is, for jealousy is the zeal of conjugial love. The reason why zeal is like a blazing fire of love is that zeal belongs to love, and love is spiritual heat, and thus in origin is like fire. As regards the first point, it is well known that zeal belongs to love; being zealous and acting with zeal means the same as by the power of love. But since on coming into being it does not look like love, but like a personal or public enemy, who attacks and fights against anyone harming that love, it can also be called a defender and preserver of love. For it is the nature of every love to burst out into indignation and annoyance, if not fury, when deprived of its pleasures. If therefore a love, especially the dominant one, is touched, a mental disturbance is felt; and if the touch does harm, it becomes exasperation. These facts allow it to be seen that zeal is not the highest degree of love, but love ablaze.

The love of one partner and the answering love of the other are like two allies. But when the love of one rises up against the love of the other, they become like enemies. The reason is that love is the being of a person's life, so that anyone attacking his love attacks his very life. There is then exasperation against the attacker, such as is the state of anyone if another makes an attempt on his life. Every love, even an utterly peaceful one, is capable of such exasperation, as may be plainly seen from hens, geese and birds of every kind. They fearlessly rise up against and fly at those who harm their young or steal their food. It is well known that certain animals show anger, and wild beasts show fury, if their whelps are attacked or their prey is stolen.

The reason why love is described as blazing like a fire is that love is nothing but spiritual heat, which arises from the fire of the angelic sun, which is undiluted love. Love is heat like that from a fire, as is plain from the heat of living bodies, which comes from no other source than their love; also from the fact that people get warm and fired in proportion to the depth of their love. These remarks make it obvious that zeal is like a blazing fire of love.