## 5 Discussion
Before any discussion on the potential future of M&S frameworks is occasioned, a discussion on the contemporary state of the industry is in order. The industry is presently undergoing a new revolution termed Industry 4.0 (I 4.0). Unlike the preceding Industry 3.0 paradigm where automation of individual machines and processes was sought, Industry 4.0 emphasizes on the digitization and integration of all the processes. To bring this about, Industry 4.0 is heavily reliant upon key technologies such as Virtual Reality (VR), Augmented Reality (AR), Artificial Intelligence (AI), and Digital Twins (DTs), among others, which are integrated through sensors, cloud computing, the Internet of Things (IoT) and, more importantly, real-time monitoring. The M&S frameworks are now increasingly being integrated with the I4.0 paradigm. The resultant framework is termed as Simulation 4.0 [17]. The primary constituents of this framework, as depicted in Figure 16, include in addition to the traditional simulation techniques, the concepts of digital twins (DTs), Augmented Reality (AR), Artificial Intelligence (AI), Petrinets Simulation (PS), Virtual Reality, and Virtual commissioning [181]. This integration between the two I4.0 and traditional M&S frameworks has been cited as beneficial for modeling sustainability [182]. The benefits cited are:  
* I4.0 enables modelers with analyzing and representation of the system at both low and high-level resolution;
* Possibility to automate the modifications like what-if questions;
* Modelers have greater power with regards to understanding and analysis of the integration of the TBL measurable factors within the system. Despite the benefits of the I4.0 for modeling sustainability, it has been argued that the M&S framework requires a major overhaul [182]. Although the I4.0 framework has provided with greater data transparency, computation power, and speed, the greater objectives of CE are far from being realized. The literature points out the following weaknesses in the current M&S framework. * Current frameworks are deeply invested in the states of equilibrium and optimality. A complex system that engenders the TBL needs not necessarily have a unique optimal state [180];
* Current frameworks are designed while assuming industrial systems and SCs are closed systems rather than open systems. There is an implicit assumption that these systems can only exhibit function and not evolution and, as a result, any structural change in the system renders the models useless [183];  
Figure 16: Simulation 4.0—Simulation approaches applied to industry 4.0. * It has also been argued that the sciences that deal with sustainability are fundamentally different than those that give rise to sustainability issues while the former are engaged with the \"causes\", the latter are engaged with the \"symptoms\". In view of the preceding discussion, it is evident that there is a manifest gap between the current state of M&S methodology and the requirements of modeling a circular economy. It is inevitable that a new paradigm of M&S frameworks is brought about. The prerequisites of such a framework can be delineated as:
* Utilizing the benefits of hybrid M&S techniques which have greater efficacy in the assessment of complex systems;
* Integration of the traditional M&S techniques with the I4.0 paradigm whereby models may have greater granularity and abstraction;
* Sustainability systems are open and complex systems that warrant constantly changing processes where the optimum point is not known in advance.