**6. Conclusions**

As a demarcation of the past, present and future of intelligent systems, a Tri-X Intelligence (TI) model is proposed in this paper to state the mechanism, factors and connotation of three main entities (conscious humans, physical objects, and cyber entities) including single-X intelligence, two-X integrated intelligence and three-X complex intelligence. Every single entity shows primitive intelligence. Two-entity integration creates integrated intelligence. Three-entity fusion generates advanced intelligence. The intelligentization mechanism of artificial systems continuously converts human intelligence to machine intelligence via different channels and interfaces. With the increasing use of machine intelligence, humans will gradually play a less significant role in intelligent systems. However, human intelligence will keep influencing artificial systems in the form of software/algorithms to drive intelligent systems. Therefore, we cannot take humans out of the systems given the accelerating development of technology. The key to success is to adapt humans to new work environments, i.e., not to replace but to enhance. According to the Tri-X Intelligence (TI) model, humans need to think more about how to collaborate with cyber systems rather than training operators to work like computers.

The proposed Tri-X model (e.g., HCPS) will integrate the intelligence in a the complex system with a combination of human-cyber-physical and machine subsystems. In future research, modeling intelligence in experiments or simulations is critical. Different cognitive architectures, such as LIDA of Stan Franklin, ACT-R of CMU, SOAR from the University of Michigan, Subsumption Architecture of the MIT AI lab, or BDI (belief, desire and intention) provide structure to create intelligent actions. Different methodologies like neural networks, genetic algorithms, simulated annealing, the Monte Carlo method and swarm intelligence are approaches to create actions that could result in intelligent behavior. The ultimate goal of HCPS, or Tri-X modelling and implementation, is to achieve effective and efficient symbioses among humans, cyber systems and physical systems.

**Author Contributions:** Conceptualization, M.Z. and B.W.; methodology, Z.N.; investigation, B.W.; resources, Z.N.; writing—original draft preparation, C.P., and S.H.; writing—review and editing, B.W. and X.L.; supervision, B.W. and M.Z.; project administration, Z.N.; funding acquisition, B.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Zhejiang University Hundreds of Talents Program, gran<sup>t</sup> number 0020886.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
