First you need to define what you mean by "complex" as that could refer to many things. A useful definition for the term will be focused on informing a decision

First you need to define what you mean by "complex" as that could refer to many things. A useful definition for the term will be focused on informing a decision. Why did you ask the question? Further, definitions that rely on one or many observable phenomena are more useful than definitions which rely on subjective experiences.