# Systemic Evaluation of PSS in the Early Concept Phase Using Graph-Based Reasoning
## 4 Solution Approach
### Evaluation Logic Layer (_EVL_)  
The _Evaluation Logic Layer_ (_EVL_) contains the abstraction of the main causal relationships considered for evaluation regarding the vague PSS concept within its life cycle in order to achieve an Evaluation Criterion (EVC). The information in this layer covers is how a certain EVC is intended to be achieved by a PSS concept (from a normative perspective) and what challenges may occur, preventing the EVC from being achieved (from an explorative perspective). The basic semantic concept for this is shown in Figure 4, where the center element is a _working principle_. Working principles are typically understood as physical phenomena (e.g., pneumatics and heat transfer) provided for a component to achieve a desired effect within an operating context according to classical engineering [62]. However in PSSs, these working principles can also be socio-technical or economic in nature (e.g., value co-creation or monetary incentives to change customer behavior) and are described on a more abstract level (e.g., \"Cars are charged by customers\") in the concept phase. Working  
Figure 3: PSS abstraction framework with different layers used to model the PSS concept. The PSS abstraction framework is based on a Bayesian decision network (round nodes = random variables with different semantic meanings in each layer; diamond node = utility nodes; square nodes = decision nodes; directed edges = causal relationships). principles consist of three parts [63] and are defined for this work in the following way: They consist of an enabling logical or physical entity that is a _PSS design element_ (e.g., a designable part of the PSS architecture that still can be vague, like \"IoT infrastructure\") that has some essential _properties_ needed to enable the working principle. Secondly, the working principle has a _life-cycle context_ covering its major phases, like beginning, middle, and end of life (BoL, MoL, and EoL, respectively) that enables or impedes its proper functioning (e.g., BoL: it has to be manufacturable; MoL: it has to be operatable; EoL: it has to be decomposable). Finally, a working principle is always considered regarding some _purpose_ it has to achieve, commonly referred to as a function [64; 65]. These desired effects are either a _change of state of an operand_, an entity on which an operation is performed [66] (e.g., how materials are processed into the final result sold to customers) or a _state change of a PSS element_ (e.g., a machine changes its state from broken to working). Working principles can also have negative results (e.g., casualties and environmental harms). The purpose and _risk_ effects caused by a working principle are causally related to other working principles through the life-cycle context (e.g., allowing electric scooters in a free-floating sharing operation to be parked on pavements (working principle) can lead to government regulation (life-cycle context), inhibiting this and other intended working principles), through the operand (a causal connection through input/output relations), or through the coupling of PSS elements (e.g., free-floating parking (working principle) may require a digital payment and tracking system for authorities to provide accurate and automatic billing (required property of a PSS element)). The causal connection of working principles through the described mechanisms (directly through input/output and coupling relationships or indirectly through life-cycle contexts) provides the semantic framework for how chains of working principles of a PSS concept are anticipated to operate together, fulfilling an EVC. In order to be able to reason about the PSS concept according to this framework, it must be represented in the syntactic structure of a graph, as shown in Figure 5. The working principles connect different purposes and risks.