• To develop software is to build a MACHINE, simply by describing.¹⁾
• A software engineer needs only make a description of the required machine. Describing the required machine is usually not a simple task.
• A system is “a set or arrangement of elements that are organized to accomplish some predefined goal by processing information.”²⁾
  • 舉例來說：學生選課系統、銷售系統或是一輛車等等
• 一個系統(System)可分解成數個子系統(Subsystem)的集合

Fig. 1: 系統與子系統

• A model is a simplified representation of (part of) a system. It is written in a language with well-defined syntax and semantics, and represents certain specific aspects of a system.
• A model is a simplification of reality.
• We build models so that we can better understand the system we are developing.
• We build models of complex systems because we cannot comprehend such system in its entirely.
• The choice of what models to create has a profound influence on how a problem is attacked and how a solution is shaped.
• Every model may be expressed at difference levels of precision.
• The best models are connected to reality.
• No single model is sufficient. Every nontrivial system is best approached through a small set of nearly independent models.
• Models are not right or wrong; they are more or less useful. ³⁾

Fig. 2: 系統與模型

• 一個特定的系統可以被各式各樣的模型的從不同的觀點呈現，例如：靜態和動態的模型等等

Fig. 3: 系統可以擁有不同的模型

• Modeling(塑模)就是軟體工程的活動
  • 塑模允許我們去使用某些比現實簡單、可靠或便宜的東西。
  • 塑模允許我們以簡單的方式處理，來避免複雜性和危險。
• Modeling的構思⁴⁾
  • 物件導向設計就是關於塑模
  • 模型中的基本元件應該描述“things” 而非”action”或“processing”。
  • Modeling allows us to use something that is simpler, safer or cheaper than reality instead of reality for some purpose. A model represents reality for the given purpose; the model is an abstraction of reality in the sense that it cannot represent all aspects of reality. This allows us to deal with the world in a simplified manner, avoiding the complexity, danger and irreversibility of reality.⁵⁾
  • So, modeling is a means for dealing with the complexity. Complex systems are generally described by more than one model from the various kinds of views. Modeling then means constructing an abstraction of a system that focuses on interesting aspects and ignore irrelevant details.