The study of complex systems, although an interdisciplinary endeavor, is considered as an integrating part of physical sciences. Contrary to the historical fact that the field is already mature, it still lacks a clear and unambiguous definition of its main object of study. Here, I propose a definition of complex systems based on the conceptual clarifications made by Edgar Morin about the bidirectional non-separability of parts and whole produced by the nature of interactions. Then, a complex system is defined as the system where there is a bidirectional non-separability between the identities of the parts and the identity of the whole. Thus, not only the identity of the whole is determined by the constituent parts, but also the identity of the parts are determined by the whole due to the nature of their interactions. This concept allows, as shown in the paper, to derive some of the main properties that such systems must have as well as to propose its mathematical formalization.

复杂系统的研究虽然是一项跨学科的工作，但被认为是物理科学的一个组成部分。与该领域已经成熟的历史事实相反，它仍然缺乏对其主要研究对象的清晰明确的定义。在这里，我基于埃德加·莫林（Edgar Morin）关于相互作用的本质所产生的部分和整体的双向不可分离性的概念澄清，提出了复杂系统的定义。然后，复杂系统被定义为部分的同一性与整体的同一性之间存在双向不可分性的系统。因此，不仅整体的同一性由组成部分决定，而且由于各部分相互作用的性质，各部分的同一性也由整体决定。正如本文所示，这个概念允许推导出此类系统必须具有的一些主要属性，并提出其数学形式化。