In a pursuit system, there are only occupying and occupied. So there are two kinds of exchange at most: occupied exchange and occupying exchange. In economy, the former corresponds to contract about commodity exchange; the latter corresponds to contract about desire exchange, like cooperation.

In science of pursuit, occupied exchange has been studied very well, but theory about occupying exchange is waiting to be established.

Occupied exchange corresponds to electromagnetic interaction and gravitational interaction in universe. For occupied exchange, receiver and releaser, similar to buyer and seller in economy, can choose expected occupied from market.

In economy, desire exchange is related with marriage, divorce, cooperation, etc. In universe, occupying exchange corresponds to strong and weak interaction. For occupying exchange, receiver and releaser can choose occupying from market.

In cooperation, a pursuer not only thinks of satisfying its own desires, but also thinks of its partners' satisfaction, so each of them is not an independent pursuer. Its nature is desire exchange between pursuers.

When desire is exchanged between pursuers, the exchanged desire is similar to mesons in universe. When the exchanged occupying is productive, it is a meson with positive charge; when it is consumptive, it is a meson with negative charge. Normally, consumer has larger happiness in society, so productive pursuer is defined as positive charge, similar to proton in universe. Most unstable energy is generated from desire exchange between consumers, similar to fusion. So a married couple is similar to a deuteron.

In a perfect pursuit system, occupying exchange redistributes occupying among pursuers. When a desire belongs to pursuer X now, but better to be free, this desire will be exchanged from X to market. For two perfect pursuers, desire exchange is always two-way: both from X to Y and from Y to X are possible. Two pursuers' cooperation is similar to a deuteron. Cooperators can not pursue their happiness independently.

For human beings, although desire can not be exchanged between two persons physically, a person can think for another person's happiness. When one meets difficulty, he exchanges desires with persons who are connected to him with desire contract, and they will help him.

Both desire exchange and commodity exchange in present society is imperfect. In society, there is cheat in contract implementation; in universe, interaction never cheats.

In a research system, when using analogy between two facts, it is similar to a deuteron too. When two facts are connected by an analogy, their knowledge must be thought together. For example, when you think that the moon moving around the earth and that apple falling to ground have the same reason, suppose you find a reason for the former, the reason is automatically delivered to the later. Unlike free pursuers, the two facts can not choose their explanations freely.

In universe, fusion provides unstable energy for universe to expand. Boson exchange is also foundation for fusion. In society and research, there is also desire exchange. So if sciences can be unified into science of pursuit, occupying exchange ought to be the only reason for development of a pursuit system. How to exchange occupying the best decides the mathematical form of strong and weak interaction.